Yup, I agree. Algea bloom. I had a small amount growing on the front of the tank but cleaned it off no trouble with a decent new sponge.
There is a product on the market that'll clear it up but I'm damned if I can remember what it's called. I'd suggest cleaning the insides of the glass in the tank and off the plants, if any, as a starting point. If you do that then you can hoover up most of the gunk with a tank vac. Then add the fresh de-chlorinated water.
Also make sure the tank doesn't get any direct sunlight. That makes matters worse. Perhaps a floating plant too if you can find one either at the lfs or online. It helps to stop the algea blooming.
Good luck.
Hugs,
P.