White Grub

Looks like a caddisfly larvae. He must have come in on a plant. I can't tell the species from the picture, but most of them are harmless. Even a predatory one would likely not be able to hurt anything, except MAYBE a newborn fry. I'm surprised you swords haven't eaten him.
He will probably die, as they tend to need cold, fast flowing water.
If he lives, he will eventually try to crawl out of the water, and molt into a winged adult nymph.