Just been up to put some CH cleaner in the expansion tank of our open
vented system. There is a layer of mould/slime floating on the surface
of the water in the tank. The layer is about 1cm thick and looks a bit
like a white/grey blanket weed.
(The tank was enclosed with polystyene sheet for frost protection but I
obviously had it too airtight - I'll cut some vents in to let it breathe.)
Can anyone say what this is? In particular could it be anything
hazardous (e.g. Legionnaire's).

Matt