these are very shy crabs. They hide alot and only come out to find food.
I would not recommend them in a FO tank, as many fish may find them food. A nanoreef would be great, and don't worry about those anemones, the crabs only use a certain species which doesn't grow in our tanks. The crabs do fine w/o the anemones.
they primarily use the anemones as food gathering and defense, the anemones get preyty roughed up but seem to do pretty well for the short term. I have a crab in my 12 gal nano, see him maybe 2-3X month. I've had him for 6 month, only one anemones left