"Martin Brown" wrote in message
...

It isn't a lichen then. More likely a slime mould and/or liverwort. They
appear when the ground is completely sodden with water and stays
permanently damp. Drought will see them off.
You could try lawn sand on it which selectively damages primitive plants
chlorophyll but I don't hold out much hope. At the moment just be glad
that your plants are not floating away.

Regards,
Martin Brown

Erm, I am glad Martin...............but I think it's only the Liverwort that
is holding them down! ;o)
Much more rain and they will be floating off into the sunset without a bye
or leave I can assure you. (

Zo