1. Take out what you can manually. Keep in mind that any that blows around the tank will attach somewhere else and start fresh where it lands. Its best to remove the rock from the tank and scrub it in a bucket or something.
2. Water change until nitrates/phosphates are zero.
3. Reduce light on the stuff if possible.
4. Good CUC to clean off the scrubbed rocks. Most inverts wont eat the stringy stuff but once its nice an short they seem to like it better.
Not much else you can do other than this. It could and probably will take months to get rid of it but it can be done!!!