Wow, that thing looks nasty! I have videos, pictures and the crab still alive in a bag floating to help ID. It doesn't exactly match up although similar from a top down shot. I did take off most its off its legs on one side on accident when removing him. That picture is under a 30x scope and he cant be more than 4mm. The shell pattern, symmetry and color is my biggest reason why I don't think its a gall. That and seeing the underside under the scope it doesn't look the same, but I could totally be wrong.
Is there a good invert ID source with taxa info and sketches available?