The photo in post #10 looks very chiton-like (note the overlapping scutes), but the distinct head / tentacles say "not-so-fast" to me...
SaltyFilmFolks is right though - a gentle touch would tell the tale. Chitons have a shell structure (though not as crisply hard as the shells of some other molluscs), nudibranchs have no shell, and very little solidity. If it's a chiton, it falls solidly on the good-guy side, but if a nudibranch . . . Some feed on algae, but many dine on coelenterates. They're often very prey-specific, going only for a single type (zoanthids, for example) or species (Aiptasia, as an example) of coral or anemone.
If this guy came out of your liverock at four months in, I'd lean toward chiton, as its algae diet is likely to be more available. (If it'd been snacking on your corals for four months, you'd have noticed, no?)
All of that having been said, have a Google at "fuzzy chiton", and see if anything familiar shows up.
~Bruce