I have got them in my shop before under the name Patriot Crab. (Cardisoma Armatum) I think from Africa.
They are not fully aquatic so give them land. THe water you keep with them can be fresh or slightly brackish. THey are messy so be prepaired to clean the tank often. They are not picky eaters, will eat about anything offered. THy are pretty agressive too.
I am not aware of these being refered to as a mangrove crab. I have seen them sold under the name rainbow crab and sometimes moon crab. THe crab I am aware of that is sold under the name mangrove crab is definetly not what you have.
Great find you have there, neat crab.....
Joel