As with any crab, they are oportunistic feeders. They will eat what they can, when they can.
if youre having issues with certain algaes that you need help with, then I am not against crabs, but to simply add them just as a novilty is not a good decission.
If bubble algae is getting out of control, by all means, get an emerald. If you have a bubble or 2, try manual removal (pop & siphon).
Same with hermits.. If youre having turf algae isssues, a few hermits will help. If its just a small patch on a small rock. Then Id look at a tooth brush and a spare buchet of salt water.
(FWIW, I have never had any issues with emeralds catching fish.)