Brown slim in my fresh water tub ATO

[donkey77]

ok iam getting red algae on my sand bed, i keep my water p in check, I have an idea where its coming from, i use ro water, new membrane, and filter. My auto top is the problem i think, i went to canadian tire and got a submersible 1/6 hp pump, i then have this in my basement, and in a 20g black plastic trash bin and my DT is on my main floor, there was this brown slim on the bottom could this be it, feeding my DT, I don't know any help