Try a stiff broom, some sharp sand, some water and possibly a squirt
of washing up liquid. It works a dream and isn't horrible to use.
Why does everyone think in terms of toxic chemicals?

Well in defence.....
The sand and stiff broom may well SHIFT the algae, but it wont Kill it, so
you will have the problem back again quite quickly,
Formaldehyde will kill the algae and spores.

Alternatively you could tarmac over the top......


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