If the covering official judges that the receiver is no longer a potential blocker, it's a foul for either holding or, if a pass is thrown during the play, illegal use of the hands *, either one carrying a penalty of 10 yards from the basic spot and replay the down.  If the safety left his feet to make the hit, I'd probably call a personal foul for unnecessary roughness for safety reasons because the player being hit is in a situation where he would normally expect to not be hit.  And of course I would call it spearing, helmet contact, or pulling/twisting the facemask if it was.

* Granted the illegal contact may not be with the hands and it may not be holding, but until the rules committee gives us something better to call it, that's what we've got.  Changing the rule to call it illegal contact would probably confuse coaches because of the NFL's 5-yard zone, but as long as it's illegal and we call it, that's what counts.