crab i.d

[inspector]

Hi all,I noticed a crab last night in my live rock,i did'nt get a great look at it.it is grey in colour with pink or red eyes its about 1-1/2" across.it hides in the corner and curls up when the light hits it.sorry no pic's.does any one know what it is and if it is reef safe,if not how do i catch it ?thanks Paul