the apostrophe would indicate ownership or possession. In "projects list," the word "project" describes what kind of list. If you say "projects' list," you are saying that the projects own the list.
Look at the alien spaceship! = alien describes what kind of spaceship (in other words, it's a spaceship from outer space)
Look at the alien's spaceship! = alien's indicates whose spaceship... it belongs to an alien
Look at the aliens' spaceship! = aliens' also indicates whose spaceship...in this case, it belongs to multiple aliens because the apostrophe is after the s
Look at the spaceship of aliens! = this sentence means that the spaceship is full of aliens