If it's available live shrimp of some sort (or stuff that looks like shrimp such as crawfish) are good starts to getting them on something nutritionally better and at the same time, wearing down their teeth decently until you can get them on clams or other marine based foods (clams,mussels,squid,etc).
Or even better would to offer it a small tank terrorist (Assorted small hermit crabs best use) and see how it reacts to that.