Goodbye to Muck Mats can only kill the algae you set your Mat on. Surface algae can float above a Mat after it’s installed. Or there’s a type of alga called “chara” (it looks like a weed) can settle on a Mat. It’s not really rooted, it just sets on top of your Mat.
For floating and submerged algae we suggest copper sulfate, a non-herbicide remedy. Romans used it in their public baths to keep algae from growing. Farmers use it to prevent algae in ponds that livestock from.
You can get it at local feed stores or buy it online. (We don’t sell it).