I don't know if they eat zoo, but my zebras have always gotten stubborn and wreckless when they get bigger. I've taken 2 back to the LFS for this reason. HTH
If they are staying in the zoos and seemingly picking" around in them.....your zoos may have detritous laying at their bases and the crabs are feeding on that.....use a baster and blow out the bases of the zoos and see if a cloud of matter comes billowing out.....if so...clean the bases of the zoos out and the carb will no longer have a reason to be there as food will be readily more available elsewhere