It's not "moss" you're referring to but 'algae.' Algae grows almost anywhere but the problem is sunlight. Do you have too much sunlight in there?

I've never used anything to treat it, if this is the case, because it won't hurt your turtles. You can buy tons of solutions at the petshop if you want to spend money.

Get some algae eaters, snails, and maybe just scrub it off now and then. I'm quite sure, however, that it's not "moss." Moss wouldn't grow under water.