If you want to catch the Damsel without tearing down your whole reef, here’ how to do it. Get a clean, clear 1 liter bottle. Cut the top off at the point where it starts to narrow. Take the top piece you just cut off and turn it upside down ( so it looks like a funnel). Place the bottom of the funnel inside of the remainder of the bottle. Secure it with clear tape.
Lay this trap on it’s side at the bottom of the tank. Insert some brine ( preferably live)into the trap. The damsel will see the movement of the brine and try to feed. The only way the damsel can eat is by swimming into the trap. Once it swims into the trap it is very hard for it to get out. Then you just lift him out of the tank.