"Beemer" wrote in
. uk:

Any advice on dealing with this?

Clean them off and decrease the amount of food being put in the tank. The
fact that they are multiplying so quickly suggests there may be too much
plankton-like foods in the tank.

How do you feed your corals? Spot feeding with a syringe may allow you to
reduce the total amount you are feeding.

They may just be a type of spiraling calcious bristle worm and not a true
barnacle--I see these guys all the time in reef tanks. They carpet the back
of my nano-cube reef. Dealing with them is the same.