The message
from Ron Clark contains these words:

I was wondering whether there was some effective commercial product
available, as this is quite a common problem?

Supermarkets sell spray products designed to kill moulds in bathrooms
and showers. I see no resson why you shouldn't use one of these,
leave for a few days and pressure hose the detritus out.

It's almost certainly an alga, and I don't know how or if a fungicide
would affect it. Just wetting it and washing it off should suffice.

--
Rusty
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