Never seen underwater maggots before....
There's a picture of a maggot looking critter top right photo if you follow this link.....
The photo you posted is not clear enough, but I'll guess it's some sort of lepidopteran
larvae that will turn into a moth. Watch the leaves on your plants, if there's chunks cut out of it, it's definitely a aquatic caterpillar.
I've only seen a few in my tank, but seems like you have over a dozen of these cocoon like critters.