IF he is happpy leave him on the sand!!! A clam that size gets most of his energy from your lights, but still needs some phyto. (so use phyto sparingly) I had a crocea start doing that (folding/pinching mantle) when I removed my polyfilter. When I replaced my polyfilter he returned to normal almost instantly. So, if he's happy leave him alone and add a little phyto.