Looks related to the porcelain crab to me. I used to have one something like this, the LFS called it a caribbean porcelain crab, however it didn't have those crazy antennae nor the hook thingys on the end of the pincers. 
 
Have a look at its mouthparts between the eyes, if it reaches out with a fan appendage (I think I see one there in the photo), grabbing at the water, its a filterfeeder and likely harmless. 
		
	
		
		
		
		
			
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