I'm trying to identify an alien movie from the 70's or 80's in which aliens impersonate humans. At the end theirfeet start to shrink and they start to melt all over, even their faces
Sounds a bit like They Live (1988), I could be wrong mind you.
Storyline from IMDB:
Nada, a down-on-his-luck construction worker, discovers a pair of special sunglasses. Wearing them, he is able to see the world as it really is: people being bombarded by media and government with messages like "Stay Asleep", "No Imagination", "Submit to Authority". Even scarier is that he is able to see that some usually normal-looking people are in fact ugly aliens in charge of the massive campaign to keep humans subdued.