This is the first year we have had a pond and it's really a learning
experience up to now. We put a small water fall in the middle of the pond,
using a number of garden rocks and directing the water from the (out of
water filter). There are 10 goldfish who seem to be doing just fine, however
one problem seems to be this dark green film that is accumulating on the
rocks, where the waterfall is.

What am I doing wrong?

Bob