A bleach or jeyes fluid mix and a stiff broom should do it, as long as there's no possibility of run off or contamination in the pond.

If you have a jet spray it would soon shift it, or borrow one if you havent got one I start scrubbing and DON'T use any chemicals what ever you do as it could run into your pond & harm your fish if you have any in the pond but if your pond is raised you could be ok.
Jet sprays are so useful, I have a pathed area and wouldn't be without mine for cleaning all the green slime and grime off so its nice and clean for the spring and summer,

A cheaper way would be to cover it with sand for a few weeks. Anything that would exclude light.

I would use a high pressure hose spray, it doesn't use any chemicals and clears all the green away beautifully.