Pa Nancy is correct, at least in my experience. When I have string algae the water is crystal clear. If there is a way to keep this stuff from blooming, I have not heard of it. In my experience it happens in the spring and early summer, before the rest of the plants have had a chance to explode, and suck up all the nutrients. Once I have established a lot of plants, the string algae disappears.
There are things that may help, such as Barley Straw, or perhaps, as Pa Nancy suggested, Koi Clay (I have never tied it). Water changes may help, but excessive changes can exacerbate the problem (depends on your water), continue your normal routine with water changes. Sunlight + nutrients = algae. If you pond receives excessive sunlight you might try an artifical shade barrier of some type. Watch you feeding practices. Overfeeding is a problem, both if there is food left in the pond and the fact that overfeeding causes excessive fecal matter in the pond. Either are an unnecessary source of nutrients for the algae. The digestive system of a goldfish is like a tube, the more you put in the one end, the more comes out the other, even before it is fully digested. Not a good idea to feed the algae instead of the fish.
Actually, although it is not very attractive, I like it in the pond in the spring. I never allow it to take over, but a mat here and there is not a problem. It is used by the fish for cover until the other floating plants provide it for them. It makes a good spawning mat for the fish and it is a source of food for the fish, particularly the tiny fry.
