I would try bleach. I'm not sure if it's bad for acrylic, it just might make the acrylic cloudy looking but you're using it as a sump so it won't matter how clear the blue acrylic is as long as it doesn't have algae or salt on it. Some have said to use something to scrap it off but if you use a scraper, you will definitely make notable scratches on the acrylic. Also, whenever I use bleach for anything for my tanks, I always first rinse it off completely and then if it's a tank or a sump, I will fill it with water and put double the amount of water conditioner in it and leave over night. The water conditioner will get rid of the bleach (Chlorine). Never had a problem. The Doctor.