Interesting. It definitely felt gritty like sand but I’ll check again. Thanks.2-3 times daily (at your stage of the SLAM) to test and replenish your chlorine is plenty adequate. The SLAM article spells this out but check as often as you like. Stay with the SLAM until you meet the three criteria that tells you you are finished. Try to finish the SLAM before you close.
If you want proof that it's dead algae, carefully try to pinch some between your thumb and forefinger. Algae will simply "whoosh"
away.......dirt or sand will feel gritty.