Blocked non return valve on caravan water system

I have just cleaned the non return valve, again, on the external water supply line on the caravan. The problem manifests itself by water running back out of the inlet when the pressure pump is operated.

The obstruction was small dark green things that look like small plant leaves. At first I thought ants might be carrying them in from a nearby shrub but the problem has occurred again after I put an insect proof cover over the inlet, when not in use.

So I assume that it is some kind of bacterial growth. Does anyone know what it is and the best treatment to keep it under control? The blockage has occures about three times in a year.

Thanks.

Cheers, Kevin.