White and stringy is often a sign of internal parasites. Although if it's moving, are you sure it isn't the water current? If not, that's a little terrifying. I've never personally had to treat IPs but I hear a lot of people say Prazi pro is what's up.
Look at the fish's vent area, if you have little, moving red threadlike objects sticking out, your fish have camallanus, which is a p.i.t.a. as far as fish diseases/parasites go
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