The “Martians” from Larry Niven’s *The Meddler *- not their real name, just what the protagonist calls them. They are basically just a lump of alien flesh in their rest state but can mold themselves into other forms as convenient, including a human-looking one. Skill seems to be involved; the more recent arrival looks oddly rounded and unfinished, while the longstanding resident of Earth looks normal except for needing a wig (apparently hairs are too fine to fake).
They are self-described as genetically engineered, so there’s no telling how much of their ability is natural.