Reading this makes me suspect I have one. I saw it one time and not long or close enough to know what it was. It looked "dusty" and when I looked up gorilla crab to see what you were talking about, I think that might be it. I have not seen it again since the initial sighting but no doubt it's still in there.
I got a pom pom crab who I never see either. I guess it's still in there, as luck would have it, the day I saw one, I did see the other one too so I know it's not just one.