[i have crabs]

sounds like ich or a similar parasite,
google saltwater ich there are lots of articles to read, its a parasite and was probably in the tank and on the fish when you got them, most treatments that are reef safe are garbage asfar as im concerned, you can take lots of different actions a couple common routs are
1. do nothing hope it goes back under control, problem is ich is still in the tank waiting for another chance to take over,
2. feed with garlic, add reefsafe ich snake oil treatments buy cleaner shrimp/fish, problem is you will still end up with ich in your tank waiting to take over again someday but better chances of fish surviving than doing nothing,
3.buy a qt tank if you dont have one yet, big enough for all your fish to live in for 4-6 weeks,treat the fish with either hyposalinity or a copper based treatment for min. 2 weeks and leave your display fishless for 4-6 weeks, this will kill the ich parasites in the tank and on the fish and will now not have to worry anymore.
ich can be reintroduced into the tank by adding new fish that have not been treated properly, or in there egg like state on rocks or corals you buy,
having a place to keep new rock and corals for 3-7 days is ideal but most people dont do that step since the risk is small