unfortunetly the crabs won't do anything about the hair algae but will help clean left over food and waste but now when you add what will help with the hair algae that invert is gonna be crab food I would check your phosphates and flow in your tank plus what are you feeding and how much? You could get a sea hare which should be not disturbed by your crabs and will eat all of your hair algae just really take the time to properly acclimate it and don't let PH, or salinity take any drastic turns or it might kill the sea hare.
p.s. add at least 1 snail per gallon like trochus, ceirth, abalone, nassirus, astreia. Stay away from chestnut snails, bumble bee, mexican turbo snails and any other turbo snails.
mr.reef24