Cropping a circle grays out the crop option

As mentioned you must have active selection to be able to use Crop function. Easy way is to Ctrl + click on layer mask to load mask as selection. Hovewer, you can right-click on mask and select Apply Layer mask what will remove mask and crop to mask. In your case nothing will visually change, mask will dissapear and pixels which are currently invisible will be cropped. 

You need a selection (or use trim if you want to remove the transparent area around your circle...)