Some of the info given here is incorrect.
GSA is generally caused by low phosphate levels. Also Low CO2 and poor water flow. Too long of a lighting period doesn't help either.
Firstly I would make sure your co2 levels is reading 30ppm and make sure your water flow is not restricted. If everything is in check I would dose a bit more of phosphate and wipe it down or add a cleanup crew to keep in check.