There's no guaranteed fix to get rid of it forever. Tanks just like the ocean go through changes. If youre not already you should start to run a phosphate remover such as phosban or rowaphos and increase water changes until its gone. Don't pull it out, you want to watch it deminish to know you got the cause nailed down. Run it in a fluidized reactor instead of a bag because more surface contact with the water that way.
Phosphate levels(even at levels that are most times unreadable by our test kits) are usually the culprits behind it.
how much water volume and size of tank and sump?
are you running a skimmer and what kind is it?
how much are you feeding?
how many fish are in the tank?
are you using a RODI?
have you checked all your parameters especially Nitrate and phosphates? (and they are all in check is not a good answer)
what kind of lighting are you running and how long are they on during the day?
how are you using the phosban media is it in a phosban reactor?