Looks like you are feeding your Sully tender new Acacia tree growth? Not sure if thats it?
As far as the algae, i would think you might want to just take a soft brush & give him a good cleaning and remove all that algae. can’t be too good to have “slimy” growing algae on a tortoise’s shell.
The only thing to worry about would be shell rot on the plastron. Sulcatas aren't prone to shell rot, but because he's spending that amount of time in the water, it might be something to check from time to time.