Found a Crab in my tank good or bad?

I had a small bit of soft coral growing on live rock, it was starting to get really big, the other day i looked at the tank it looked like it had been half eaten. also i had bubble algae about 6 now its gone . To day i seen a crab i did not buy he's not to big, is he to blame for my soft coral, or could it be my fish only have 2 clown and 2 damsel's. should i get him back to the shop

 

We need pictures. And this should be in ID This. Get out the camera and get a picture of the crab!

 

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At first glance it looks harmless (I'm not seeing hair, sharp claws, or other concerning attributes). You might want to QT it to get a better picture.

 

Try observing it yourself or get a closer pic.
Pointy claws:bad
Non Pointy Claws:good

 

ill get a better pic and post it, cant find him at the mo but ill post it over the next few days thanks

 

i think he has pointy claws, didnt get a great look tho he was in and out of a hole, will post better pic's soon,