He's become melanastic, so there will be some darker/gray areas. Clean him up and give us a good look
For cleaning algae on the shell, I usually gently scrub, rinse, wipe and repeat. Betadine is good and a good surgical scrub should have chlorhexidine (Hibiclens). Just be aware that Betadine might cause stains if you spill it and Hibiclens is a bit soapy and can make the turtle slippery before you rinse him.