I pulled one off a zoa the other day. It was stinging one of the polyps which kept it from opening. After pulling it off , I looked at it under the microscope and at the end of the stalk it looked like a tiny anemone.
Colonial hydroids which DO sting. You can place a dab of gel super glue on each which is a risky pain. Best bet- Simply starve them by directing water flow towards them, Not at them and they will starve and vanish