Crabs are opportunistic feeders if not outright predators and as such will eat whatever is easy to get to. Just because they are well fed does not mean they won't eat the things you don't want them to. Especially as they get older, larger and comfortable with their surroundings.
Here is a blurb from wetwebmedia.
"Mithrax sculptus, the Green/Emerald Crab. Tropical West Atlantic. A noted eater of algae (even Valonia), but can turn into a fish eater... some references state to 2.5 others to a maximum of four inches carapace width. Now in its own genus Mithraculus. Family Majidae".
I am only saying here, do not take the word - experience of any of us here on the site. Get yourself some good scientific information on what you want to keep and decide for yourself if you want to try to maintain any of the wonderful, and not so wonderful, creatures available.