Just saw this guy for the first time today. I've only added one frag since last November and it was really small, so I doubt he could have sneaked past me. So I have no idea how long he's been here.
Well they usually look like whatever they eat. If you have an corals that have that coloration and look like him then that's its food source and it'll eventually starve after it consumed the coral.
If not then it's eating something else of no importance in your tank and might just be a part of your invert crew.
Ya I gotta agree, kill it or trade it in for some lfs credit, if it wasn't eaten in my tank and I found it....I woulda flushed it depending on the time.