I just had this problem recently. I did about a 40% water change and removed 1/4 of my bioballs and replaced with LR
rubble. I also added iodine, purple up and calcium. I can scrape it off my glass and two days later its back. It is also in my two dead spots on my sand. I have had this happen before and I think it is a chemical imbalance. Nitrates at 10ppm, NItrate 0, ammonia 0, calcium 400, ph 8.0-8.2 range. Temperature is about 78-82 tops and lights are on 10 hours a day. I get brown, red and some green slime algae.