Oh...I was just looking at the glass.
A few bullet points that may help:
Manually remove as much of the GHA as possible, daily. That back wall is a good starting place! You have to keep on top of the GHA or it will come back.
Do water changes and try to syphon out the purple stuff along with the cyano. Vacuum the substrate.
Peroxide can help with GHA, but do some reading on that prior to use!
Do research on ReefFlux and how it will affect your corals. I'd try other methods before chemicals.
Others have given some good advice above as well (i.e. flow). Try lowering the powerheads and get water moving down low.
