I'm curious about the water temperature. Algae can grow in conditions that we don't expect. There is nothing particularly strange about it although it is interesting. The fish may nibble on it. If it gets out of hand, sweeping it with a bowl brush or pouring some H2O2 directly on it will control it.

I have seen it every winter and I think the fish appreciate it when they are still somewhat dormant and can nibble it here and there. I think it is a good thing...until the spring clean.

I have string algae also, and I pull out any that is free floating to keep it from clogging the filter tubing. Otherwise, for the most part, I just leave it alone. It keeps the water crystal clear and absorbs nurtients that might otherwise foul the water while the filter bacteria are dormant.

I wouldn't worry at all about the hair algae being in your pond. The algae will help to filter the water and it is a perfect first food for Koi and Goldfish to help start up their digestive tract for the coming spring.
Any type of algae starts up due to feeding Koi the wrong type of food, too many fish, feeding too much, filtration system not large enough and ponders who do not make monthly water changes.
We don't have plants in our ponds so having a good bio-filter helps to keep our water clear and healthy for our fish.
We've been making our own bio-filters for years and we help anyone make one. I've also got a diagram.

Yours Koily, Lorraine

We examined some of the winter algae carefully yesterday. It is much more chartreuse than the summer algae as I had mentioned. What I had not noticed until yesterday is that it is less slimy, less stringy and all kinky.

I had tried making a planter from the summer stuff by wrapping it around a plastic pot and letting it dry and then putting some soil and seed in it. My pot made of summer algae rotted before the seed amounted to anything worthy of mention.

I will try the making a planter of the winter algae and report back in a few months. In addition to just tossing it in the compost, there must be some use for this most curious stuff.

I hadn't heard of making string algae into pots before...what a good idea. I could see where it might need some wire support or seriously high pressure like they use to form peat and cow manure pots. But even just lining a pot with string algae I bet would be good for most plants.