It's a brand new setup so I think it's an algae bloomYou say you're cycling the tank. Is it a brand new set up, new pump etc etc etc? If it's all new i doubt it would be built up gunk in your lines but if you're just starting a fresh set up using gear you've just taken off another tank then it could, as previously mentioned, be a release of gunk from your return line. It happens to most of us i think, unless you clean your return lines every week. It does clear, it just looks a bit unsightly.
I bought some api quick start and put it into the tank, it's all cleared up now, and there is a rhino pleco and a regular pleco in the tankPlanaria was my first thought as RD stated..what are you using to cycle? If it's some sort of meat, meaning market shrimp/tilapia then I would say planaria
Hello; If you wrote something it is not showing up. Are you using a white font? That does not show up for me.