I'd agree that it is cyanobacteria. Cyano
feeds off of nutrient rich water. The cure takes time. Constant changes with ro
water, or distilled water at best. Lessen feedings, I only feed my reef tank twice a week. If it is bad, you can use a product called red slime remover, but it will come back if the issue isn't addressed like I listed above. Keep in mind, cyano
also thrives in low flow areas so increasing the flow will also assist with this, but the nutrient issue will still remain unless addressed.