I think that's just a mold of his face. It's not like it actually moves, it's like a plastic shape of his face that's being rotated under the ick.
According to Frakes, he was actually pulled into that muck.
Oh, he was indeed pulled in. But when the face comes up, rotates, then sinks back down, that wasn't Frakes. After all, as it sinks back, you can see the goo coming up in the mouth, and unless Frakes was willing to swallow some and then regurgitate it on cue, that's not the sort of thing a person can do.