yep, that sounds like what i had. i just cleaned everything in the tank with really hot water and let my turtle dry out for a couple of hours. i think it might have been because i moved the tank near a window but it also might be worms inside your turtle. i would take a sample of his poo, you're going to have to do this if you need to take him to the vet anyways, and carefully inspect it with a magnifying glass and a toothe pick, if you see those worms in the poo then your turtle has worms and you'll NEED to take it to the vet asap. if you've cleaned everything, i mean everything gravel, plants, filters, heaters, etc. and your turtle still acts normal and eats normal and you don't see anything wrong, than i wouldn't worry. this is what i did when my turtles tank had this problem and it has been a couple or months and i've not seen another worm and my turtles behavior is still the same and he's still active. BUT IF YOU WANT JUST CALL A VET AND TAKE A SAMPLE OF THE WORMS AND YOUR WATER WITH YOU TO THE VISIT!! let me know how it turns out!! thank you!