The crabs may not be the only problem but if injured they will definately make things worse. When injured you must be very sure they get adequate water flow to help prevent infections (mainly brown jelly disease) but not too much flow to cause further recession. Also be very sure sustrate never gets lodged between the sclerites. That will increase the possibility of infection and impeded new growth.
If at all possible depending on the size of the coral, get a berry basket like the ones used at the local market. Make sure the holes are small enough to prevent further damage from vagabons but do not impede proper water flow. Clean it well in very hot water without using cleansing agents, turned upside down and place over the coral. You can probabley fashion something yourself but this will give you a place to start. Keep the nitrates as low as possible through water changes and add a good marine
liquid vitamin to the water regularly that is high in vitamin C. This will help reduce or eliminate the instances of infection and aid the coral in new growth.
If you have any other LPS
corals in the tank be watchful for stray polyps. Some LPS
tentacles have a habit of breaking free from the parent coral and carry very powerful nematocysts. If these land on your coral, they can also be the cause of the flesh errosion. If removed in time, the coral will often recover quickly water quality depending.