Nah I'm not positive. It's hard to tell from the photo. I leave the algae in my ponds- the fish eat it and since I have so much top cover it never becomes a problem.
It's quite possible it's an algae or it could be something else- are you missing any fish? Might be growing on a dead fish or even leaves.
Removing it is up to you- algae normally doesnt harm fish at all if kept in check/balance. Some- well alot of algae can explode from even the tiniest piece. If you scoop it out- try and get a picture! lol Algae makes great fertilizer- plop some on a plant. Maybe grow it in a five gallon bucket with water in full sun for plant food.
Raising KOI
Have you tried this. I saw in a thread that you can feed fish cooked oatmeal.
The food I am using now just floats and the fish will not come to the surface to eat it. Guess because of the surface tempture. Anyway I tried it and they ate it all. It sinks to the bottom.
Seems like I was trying cooked oats last year. I don't remember if they liked it though- are yours doing well on it?
Only tried it this one time. Wanted something that would sink to the bottom. My fish are still active and eating. Just not surface eating.