I have the same thing with a lot of the compost I have used on the Dahlia
tubers, it's the precursor to Moss, starts like a green algae, then it will
start to grow into moss.
I don't know why it is so predominant this year, same compost I have used
for a few years, and the conditions are the same.

--
David Hill
Abacus Nurseries
www.abacus-nurseries.co.uk