Chris Hogg wrote:

The blanket weed (and any other algae in the pond) is being fed by
nitrate in the water, just as any other green plant needs nitrate
fertiliser. As others have said, this can come from several sources.
One not mentioned is food given to goldfish. Goldfish crap contains
ammonia, which is converted to nitrate by bacteria in the pond. Excess
food is also ends up as nitrate. It is quite possible to manage a pond
without getting masses of blanket weed, so long as you are aware of,
and minimise, sources of nitrate. Avoid feeding goldfish, and don't
use ordinary garden soil for planting marginal plants in.

And if you haven't got any already, introduce some water snails.

--
Rusty