Mural on the ceiling changes - why?

the WHOLE ATMOSPHERE (within and without the Pyramid) changes once the Humans arrive. :)

it's a change in atmosphere, the room had been sealed for a very long time, the murals turn to mush basically

yes, the atmosphere changes, but there's a deeper meaning than that. It's meant to symbolise a change on a different level, fitting the philosphical / religious themes of the film. Remember this 'room with the big stone' is a temple. It has an altar, and an alien figure in cruciform position on the wall. And a mural reminiscent of the Cistene chapel (in a tongue and cheek way, because there seems to be a facehugger on the Engineer, and supposedly the Engineer's maker is happy with this. (the Engineers seem to be sexless worker drones, and there may be a higher caste version of them in the next film). The change in the mural probably meant to signify a change in that the things created by an Engineer have come looking for questions, and believe they deserve answers, (rather than simply being subservient to their 'god'.)

The "black goo" material (let's call it "primordial slime") is charged to be attracted to, and assimilate, DNA. This primordial slime is not just in the jars, but used as a component throughout the ship. Thus, the entry of new, living genetic material into the chamber alters things. . . moves them. Either that, or the acid just kicked in and nobody's going to be useful for the next twelve hours.

The chamber was hermetically sealed and devoid of oxygen. When David opened the door, allowing oxygen, humidity and an increase in temperature to enter the ampule room, everything began to mutate, ooze, boil, etc., giving the appearance that murals were moving/changing. I do like, however, the deeper meaning/symbolism idea above. I hadn't put it together that the ancient xeno was in a crucifix pose.....and this chamber was last open to air 2000 years ago whilst on Earth crucifixions were all the rage.

An environmental change, but if you ask me it was also a trigger for unleashing the goo.

I think much has been read into this movie, I think the mural and statue are twisted versions of the cistine chapel and the crucifix. It seems to be in or near the weapons storage bay for the black goo. It was all 2000 years old based on what the crew tested for, that means the engineers decided to wipe out humans around the time of Chirst. During that time, religion shifted away from worshipping men from the heavens like the "Titans" and more twords universal Gods. That could have been a real "no no" becuase in this movie "The Titans" were our real Gods not some "Universal God".. so I caution against reading too much into this story, it could be that our Gods decided to wipe us all out when we stopped worshipping them around 2000 years ago, perhaps this showed to them that we were an inferior product that would never evolve into Titans and we were making inferior copies of ourselves to add insult to our Gods, if you remember that the sole suriving engineer woke up in a good mood until he saw that android and he went nuts! I guess we were a huge insult and disappointment to them. I dont think mankind is some kind of threat to them or anything like that, I think its just that we didnt measure up to what was expected.