No acid for blood...

Good question.  I suppose the easy answer is it's make believe, so the writers really aren't bound by logic.  But you could make the case that the goo transformed its victim (worm, Fifield), and also transformed the spores.  Then the infected spore had a tertiary effect on humans.  They could have went any direction here.  Maybe they ran out of acid blood props LOL.

I think its just a sign that THOSE working on the Project have no real interest in LORE and whats laid down prior...  why things are such a mess to be honest!

I agree it seems the Neomorphs dont have Acid Blood, or certainly its not as concentrated as the Xenomorphs, the Xenomorph Face Hugger also appeared to have less of a concentrated Acid as the Alien one.

The Hammerpedes had Acid Blood, in the drafts Fifield had Acid Blood, and i think the Trilobite/Engineer scene is not the full one even on the DVD extras and that the Burn on the Engineers Face was originally from Acid Blood.

Its just a bit of a inconsistency really, we also have to ask how David could dissect all those Black Goo related Organisms with no Signs of Damage from the Acid Blood....  so maybe the Acid Blood is something ENGINEERED into the Xenomorph, and the Worms/Hammerpedes once again become another oversight?

Because reasons. Not everything has to have acid for blood.

My guess is that it is an earlier version of the Xeno, they are not there yet. This is why it does not bleed acid for blood. Another answer is that they did not think a lot about it.

David told Oram:

"The pathogen took so many forms and was extremely mutable. Fiendishly inventive, in fact. The original liquid atomised to particles when exposed to the air. Ten years on, all that remains outside of the original virus are these gorgeous beasts. Patience is everything.

From the eggs came these parasites, shock troops of the genetic assault. Waiting for a host, entering the host, rewriting the DNA and ultimately producing,  well, these enviable unions. My beautiful bestiary."

David does not say how we get those eggs containing the motes. Are they a by product of the pathogen on the local flora? And if the motes rewrite the host's DNA the result should not have been a mutant host instead of a neomorph? 

Another question is how come we don't see any hammerpedes on planet 4? Why the pathogen killed all the "meat" there while on Prometheus mutated the worms?

Why the heck didn't Holloway get a backburster after ingesting the pathogen? Is it a condition for the host's mutation to swallow the pathogen in its liquid state?

The neomorph clearly did not have acid blood as the one shot in the Cathedral was lying in a pool of blood...

I think its one of those things that leaves for for debate, it appears they dont quite stick to what Prometheus had shown, it appears the ideas had changed even in Prometheus by the time we got the movie Edit... which seems to indicate that only the Hammerpedes had Acid Blood, yet earlier Drafts we see Fifield has Acid Blood,  and it appears the Engineers Face Burn was likely from Acid Blood of the Trilobite but this was changed with the Edits and Re-Shoots to be from the Juggernaught Crash.

When looking at Alien Covenant, it appears those Urns that was dropped had either been Primed in a Different way so they was activated to explode and in effect the Weapons Contents had a different effect compared to contamination from a NON-Weaponized/Activated Urn.    I have before mentioned the Atomic Bomb Analogy.  Were the Urns have a number of components and so LV-223 Outbreaks and Infections are like being effected by the Radiation/Leak from Uranium and Plutonium Material.  Where as Davids Bombardment was like when a Nuke Explodes and the Uranium and Plutonium are used as Fuel to Make the Powerful Explosion that Incinerates those in the Blast Radius.

Where those out of the Blast Radius suffer instead the Radioactive effects of the Uranium and Plutonium.   The other explanation is Davids Urns hit a Force Field that caused them to Explode and have a different effect, or indeed this was one Protective Element the other maybe after the Infection some Weapon was set off to Eradicate the Infection by unleashing a Burst of Energy that Burnt those Engineers and prevented any Mutations.

The way i look at the Neomorph Spores is that when the Juggernaught Crashed, some UN-activated Urns had came loose and Spilled the Black Goo to infect the Water Supply that was running down the Mountain were the Juggernaught Crashed, this infected the Water, which in turn infected Spores Growing from Mold near by Water Supplies...    Fungi/Spores are not Plant Life but a type or Organism so they can be infected by the Black Goo.

I would say we would only had seen Hammerpedes if any natural Worm like Organisms got infected with the Black Goo Directly from a Leaked Urn and not Exploding one.   It appears various life would have been infected by the Black Goo via leaking Urns in the Crashed Juggernaught and maybe from effects of Particles from the Bombardment that escaped the Blast Radius.

Holloway Question is a interesting one, which has a number of answers we could look at, especially if we look at the unused Props for his infection.....   We can assume the Back Burster etc is result of infected Fungi/Spores.   It appears Holloway was infected and he would have had tiny Parasitic Worms Multiply inside his body and they would begin to consume his Genetic Material overtime.

I've got to catch up.

"The other explanation is Davids Urns hit a Force Field"

That sounds very reasonable when reconciling with what we saw in Alien: Covenant BigDave.

"Another question is how come we don't see any hammerpedes on planet 4? Why the pathogen killed all the "meat" there while on Prometheus mutated the worms?"

Excellent point daliens.

There was an effort to showcase the facehugger bleeding acid when cut attempting to save Lope but I don't remember the floor collapsing from acid under the shredded neomorph Captain Oram let his frustration out on. 

You are exactly right hox. It is because reasons shared with a list of people that isn't us.

Thank you for touching on this again ninXeno426.

No problem Ingeniero.This has been bothering me for a while now.

We can maybe assume the Bio-Mechanical Evolution of the Xenomorph would lead to more Potent Acid?

But then the Face Hugger Acid in Alien Covenant is maybe just another Oversight