The fuzzy algae that grows on the sides of the pond, rocks and planting basets
and doesn't get more than, oh about a half an inch long, is substrate algae and
it is good for pond balance. Leave it be. If it is long and flowing then we
call it string algae and it isn't as popular. Sometimes it will show up in
colder weather, especially in early spring. Koi seem to enjoy grazing on it as
they aren't being fed then. Overall if it is getting to be too much just gently
remove it, twirl it around a stick or some kind of brush or small rake.
All algae thrives on sun, fish waste, fresh water, rotting plants and blown in
dust and dirt.

kathy :-)
zone 7, SE WA state