Logically you should keep the camera away from water and ensure that the there is no high nitrate in the water. Algae will grow on wet surface with high nitrate and plentiful of light.
Prevention of algae include apply algaecide or get algae scraper which you can get from major plant nurseries and aquarium stores. But how these chemical treatment is to apply on camera and lenses is still unknown and require more citation.
Keep your underwater case well maintained so that it is water tight. Should keep out water, algae and fishes.
Once water is inside just take out the cam, throw away the body and bring the lens to service center or bro 'fatigue' here in CS. I'm sure he would be interested in such a case. Something different, not the usual (boring) fungus cases.
Just wondering whether algae on lens surface can be used as green filter, kind of 'natural light filter'?
You could try coating on the glass filter (or housing) using invisible nano LCD protector liquid, then top buffer with high quality showroom polymer car wax. That should separate the water from the glass surface for a week of submersible but the interval (weeks or month) between application is unknown.
For plastic surface, you can try apply a thin coat of clear grease used in water components, that should prevent algae.