Flesh suit

A flesh suit concealed the shape of the being inside, "a perfect disguise". Flesh suits had the ability of appearing to age naturally, growing alongside its user. All three members of the Common Men, unknowingly Byulnians, had flesh suits on their entire lives. The seal for their flesh suits was at the top of their necks. (AUDIO: 1963: Fanfare for the Common Men)

Behind the scenes
Raxacoricofallapatorians' skin suits are referred to as "flesh suits" in TV: The Lost Boy. The two are very possibly the same technology; with Raxacoricofallapatorians, though, the seal was on their foreheads.