Most of the time, emerald Mithrax crabs do very well with seahorses, particularly if the Emerald crab is small and there is abundant algae in the aquarium for it to eat. Will Wooten lists them as a "2" on his seahorse compatibility guide, meaning they are generally safe with seahorses with the rare exception of the occasional "rowdy" individual, and I would agree with that assessment.
Emerald Mithrax crabs are primarily herbivorous in nature and are generally shy and inoffensive in the aquarium. I would say that 9 times out of 10, Emerald Mithrax make fine tankmates for seahorses and cause no problems at all. It’s just those rare exceptions and uncommonly cantankerous individuals you must be wary of, sir. Even the gentle Emerald crabs can very occasionally become problematic if they are not getting enough vegetable matter in their diet, in which case they may become opportunistic omnivores and are no longer averse to adding a little meat to their diet should they get hungry enough.
Remember, crabs and crustaceans in general are opportunistic predators that are liable to attack anything they can overpower. They may be entirely peaceful and inoffensive when they are small, but even a small crab can cause a lot of trouble as it grows. They may double in size following a molt (i.e., ecdysis) so they grow surprisingly fast, and even a tiny crab that’s completely docile at first can grow large enough to turn predatory almost literally overnight if it’s a species that reaches a respectable size. One day it’s a miniature crab that’s cute and entertaining in its own bumbling sort of way, and the next day following a successful molt, it can become a dangerous bully that regards its tankmates with a culinary eye.
The bottom line is that it’s probably safe to keep the emerald Mithrax crab in your seahorse tank as long as there is algae for the crab to eat and it’s a relatively small specimen, Don. But if there is not algae for it to graze on or the Emerald crab is starting to become sizable, then it’s best to play it safe and relocate the crab to another tank.
Best wishes with all your fishes, Don!
Pete Giwojna, Ocean Rider Tech Support