How often do you feed? What do you feed? What type of filtration do you have other then the HOB
that I see in the pics? How often do you do PWCs? What source water do you use for top off and water changes?
What are your water parameters like? Ammonia Nitrate Nitrite?
From what I see it looks like cyano
which is feeding off excess nutirents in the water. You probably have high nitrates and that is the cause of the issue. Where those nitrates came from is the question. Using RO
water is the best option for elimnating excess nutrients from your source water. I would suggest doing some large (30% +) water changes over the next several weeks. Doing regular PWCs will help keep the excess nutrients and nitrates in check. With that said you need to check your source water for nitrates and phosphates as both will lead to algae issues.