That would be Azolla caroliniana, a native floating fern.

It probably arrived on the foliage of migrating birds.

Wherever there is a fertile pond, something is going to invade, colonise and try to hang on to its cosey place, a reliable fertile pond is a happy place for allsorts, whether it be algae, duckweed, cattails, azolla....

Just as it can suddenly be there it can be gone with drought, floods or excessive heat going to extreme

To inhibit it, try keeping water levels high so the next storm flushes the bulk of it out. Or drain the pond long enough for it to dry out.

Mechanical or chemical options may be more bother than they are worth. If you can blast spray to one side and dredge it out, it makes a useful mulch which gardeners can use.

When it masses up, azolla shades and cools water but can become dense enough to subdue light to submerged aquatic plants

Thanks so much for the response. I can always depend on the folks on GardenWeb for answers.

Wish I lived close to you. I would love to have some for my pond. I put it in every year, but once winter comes it's gone. I can't put it in my big pond cuz my koi think it's a delicacy. Sometimes it takes over my top pond and the waterfall, but I love it. I pull it out when it gets too invasive and throw it into the big pond for a koi treat.

Jenny

That's odd, I've never seen azolla that color, just the green. You could introduce some grass carp to the pond to eat it. A lot of fish hatcheries sell the sterile carp, so you don't have to worry about introducing a non native fish and having them ruin your whole state.

Not all fish will eat it.

Mine turns red like that in cooler weather.

Paula,

You can sell it on Ebay. I buy it every year to add to my pond, but as Jenny said, it dies off in the winter. I did put some with a couple of water hyacinths in an aquarium this winter. I'm hoping it will survive til spring.

If I have algae, a large baggie of fairy moss (as I like to call it) will clear the water up quidkly.

It turns red here in the fall with the cooler weather.

Anne