We need your water parameters to help. My guess is that both nitrates and phosphates are up. A series of water changes are in order. Syphon out as much of the cyno as you can. Slow and steady wins the race.
I'd go with water changes and get new tests kits, cut back a bit on feeding, cut back on the lighting time. Take a turkey baster to the rock before you start the water change and try and suck out as much of the cyno as you can. A 20% water change at the same temp and salinity would be a good start.
I'm sure others will chime in, but you need to consider getting a RO/DI filter and not use tap. Do you know the TDS of your tap water? It may be that your excess nutrients are coming from there. A LFS should be able to sell you RO/DI until you can get a filter.