It looks like an algae bloom. The whitish area at the top is probable where your lighting is brightest.
You could try restricting the light and making sure no direct sunlight hits the tank. This would be slow but might be effective.
You could consider a total black out for five days, completely blacked out, no lights no daylight. Don't even feed your fish. The algae needs light to survive.
Or get a UV
steriliser, but that's an expensive option. The UV
steriliser does have other advantages in the long run, reducing water bourn bacteria and pathogens.
Personally, I favour the UV
, but I'm biased. My daughters bought me one for Christmas.
Let us know how you get on.