Reefing, mine came in on the original rock, which has been in there a year now. I noticed the other day there are a few on another rock now. So they're spreading, just not fast.
Seagull, you can try trapping it, which you'll probably want to do since it sounds hairy. Take a clean glass and put a piece of food in the bottom, and tilt it against the rock you saw it go into. It'll crawl in but can't crawl out. Or you can use a
bottle trap. Once you get it out, you can take good pics of it & get an ID.