Crab ID please.. Hitchhiker and possible murderer

[wnehez]

So My Emerald crab molted a while ago and then a day or two later I found him dead. I thought it was from my mistake of trying to clean the sand and mixed it up alot and caused chaos through my tank. Just the other day, I saw something move. At first I thought hmm maybe it was the emerald crab and he just moled twice and he actually survived... But then I saw this one again. He jus peeks out of the rock he lives in. I cant get a pic because everytime he gets far enough out that I could almost get a good pic, a fish swims near him and swoosh, back into his rock he goes. I got a good look at him tho. He seems almost purple with orange claws. Weird. I googled it and couldnt really find any pics of what I saw. I was wondering if anyone saw or has one of these and knows what kind of crab he is... I dont want him to be a bad crab and eventually catch one of my fish or kill other creatures in my tank...

[thegrun]

Positive crab ID is difficult with a picture and impossible without one. When in doubt with crabs, it is best to assume the worst and remove him from the DT. I know, it will be difficult to catch, but the death of your Emerald Crab is strong evidence the crab is not safe.

[Sugar Magnolia]

Place a tall smooth sided glass, baited with a small chunk of fresh seafood tilted against the rockwork where the crab lives. Do it at night as the majority of hitch hiker crabs are nocturnal.