Apologies for the poor quality photo, this thing is tiny (about 5mm across) so it was hard to get a shot of it at all. It’s purple (photo colour is accurate) and lives in a tiny white tube. It got spooked when I turned the light on and retreated, but then came back out. Might be nocturnal because I’ve never seen it before and I’ve had this piece of live rock for at least 6 weeks. Grateful if anyone can tell me what it is and whether it’s good or bad?