This sounds like you may have mustard algae as that is yellowish/brown and looks like dust or dirt. Can you post your water chemistry levels (FC, CC or TC, pH, CYA, TA, CH, Temp.)? It seems to take a higher level of disinfecting chlorine to prevent mustard algae (close to the Max. FC ranges in Ben's chart. It also takes a much higher level than the Shock levels to get rid of it. If you post your numbers, I can let you know what shock level will likely kill this stuff and what FC level you need to maintain to prevent it from returning.
[EDIT] What kind of filter do you have (cartridge, sand, DE, sand with some DE added) and how long do you run your pump each day? [END-EDIT]
Richard
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