Not very environmentally friendly but a good dose of Hypochlorite in a pump spray bottle will all but dissolve the shell. Follow up with a stiff deck brush and pressure washer but take care as its basically strong bleach!!
I would have plenty of freshwater on hand or a hose to dilute and wash down.
I used a Balco scrapper on one of mine and literally shaved them off. Didn't seem to do much damage to the hypalon. Had to be careful with the sharp edges of the tunsten blade as it could have ripped into the fabric. If you use a pressure washer be careful with the nozzle setting as the needle jet function can lift the outer coating layer of the hypalon and wreck the seams.
