Sure looks like FA mats. Esshup (of course) is right on the money, with treatment. Lots of threads on FA management in the forum too. Breaking your nutrient wash in cycle is also helpful. I started using biochar granules in erosion socks, wrapped around my intake pipes, to bind up and filter out some of the nitrogen and phosphates coming from Ag fields across the road. Has really helped. I still rake FA out, use pond dye to keep light penetration down, put down granules and spray top mats too.

"He who thinks he can and he thinks he can't, they're both usually right." - Wyatt Hendrickson, 2025 NCAA Hwt Champion