Strange thing coming out of bloodfin tetras mouth?

[Big Boy]

I changed the water I was worried and I was suppose to change tomorrow anyway

 

[Autumnsky]

Vacuuming the substrate would be ideal. Water changes are usually always a good idea when something goes bad. It is unclear what the little guy has/had. With parasites, the life cycle is usually one of see them and then you don't but they aren't dead just in a different life cycle stage.

Keep watch for gill flukes, Anchorworms, any wiggly/flat worms at the eyes, mouth, vent and gills, on body of course - pretty much everywhere. Maybe a magnifying glass could assist.