Ive had this.
I rinse it out w hot water first
then a little bleach and hot water , about a cup total.
I put my thumb over the valve area and give it a good shake.
I don't want bleach to damage the rubber gasket , if it might.
Ive used a long thin brush to get some of it out in the past
the reservoir get sun in the afternoon
after its ' clean enough '
I cover that 1/2 w a folded ' tea or kitchen ' towel
that seems to take care of the problem
and I take the towel away when using the CSO.
as mentioned above , I think its algae or the like
not mold. mold generally floats on the top of liquid
