Green Mithrax Crabs are commonly called Emerald Crabs, and Red Mithrax crabs can be called Ruby Crabs, or more often just Red Mithrax crabs.
I didn't used to add iodine supplement to my system, but when I finally did (Tech-I) my Sally Lightfoot molted the next day. So, I think crabs do need iodine to molt, too.
'Ric