Does it have the red ear on the side of its head? It's hard to tell with the algae but it also looks like it could be a map turtle with it's distinctive marks on it's head. Very well could be a RES.
Not much to go by with the picture, but overall looks definitely like a map turtle. Serrated rear marginal, overall more oval shape, and many small stripes between eyes. Probably common map. Graptemys geographica
Did you end up picking it up and finding out what it is? I'm curious. Definitely a type of map, given Mark's diagnosis of what we can see. I'm just curious if it's a Florida native or if it's a captive raised transplant.
I'm actually thinking it is indeed Trachemys scripta. What little we can see of the shape of the head, the markings on the head and the bit of markings you can barely make out between the thick algae, do look more Trachemys than Graptemys. I also think the "serrated" rear marginals are simply scutes that need to be shed.
I think this is a Red Eared Slider, Trachemys scripta elegans, but would prefer to see another pic or two before I say for sure.