I have an above-ground pool (27' round). Started to get mustard algae which I've been battling for a couple of months. BUT now the algae has adhered to the bottom of the pool in what I assume is a "cocoon" of calcium. Feels like sandpaper and WILL NOT BUDGE unless you literally scrape it with hard plastic. Even a wire brush barely dents it. Is pretty much coating the entire bottom in a yellowish-brown thick scale (some areas worse than others).

I've drained the pool down to about 4 inches with the intention of scrubbing the heck out of it but this just isn't working. I have no idea what to do...I'm literally on my hands and knees working on it inch by inch. Is there something I can add to the remaining water that will soften this stuff or should I just fill it with gasoline and set it on fire?