I can definitely chime in here because I have been thru the same thing at some point!
Getting rid of the gravel will help with the green color because the gravel tends to keep/retain the turtle waste & lots of bacteria. 14 yrs ago I had gravel in my turtles' tank and no matter how much you try, the water is never completely sparkling clean & always looks cloudy or green. Gravel is great for fish, but sucks for turts since the waste/algae/etc. tends to build up in the gravel & no matter how many water changes you do, the water is still dirty looking! If you want some rocks in there, go for some larger river rocks to decorate the tank. They are easier to keep clean & won't make the water cloudy/green.
Also, as someone else mentioned, the gravel isn't good for the turts anyway since they eat it.
