Ok, that's definitely one of the keepers IMO. I had one very similarly colored...that's the BIG one I had. I never could get decent pics of it...almost always inside a large rock...(no, don't worry about it being in the rock...it can get out if it wants to...
From the description, yours probably looks similar to this:
I was a good "host" in that I target fed my brittle and serpent (both in the same tank) bits of shrimp or silversides when I fed my anemones...usually 2X weekly.