I have been in the hobby for 10 years and have always had them. You will never beat them unless you start fresh with dry rock and sand. They attach to corals and equipment.
The best method I found dealing with them is a 3 watt laser. I feed the tank and look for the strings then zap them with the laser for a few seconds and hear them sizzle and pop watch them turn white within a few days.
At this point I know I will never beat them but I have been able to control them so they are not so unsightly.
Good luck and hope you find a solution!
What laser should I buy? I want a huge one to blow them up!! Money is no object