Algae growth on panels

I have panels on 2 roofs, facing SW & NE approx. South Wales UK, based.
Installed in 2012, they have remained clean since then, barring occasional sandy rain events, which soon cleaned off in subsequent rain periods.

In the last few months, I have noticed what looks like (from the ground!) an algae type covering on only the north - east facing panels.
My question is, how can I remove this before it builds up to the point where I get no output from these panels.
We have had some very heavy rain showers recently as well as a couple of heavy hailstorms, but this doesn’t appear to have shifted any of the algae.
Any ideas what this might be & how to remove it? Thanks for reading this.

You can use something like this:

and for the algae:

You get the idea :slight_smile: