i really think thats what it is i have 2 of them chasing each other around and i think the male has this white thing on the anal fin. Looks kinda of like the guppies( just doesnt function the same, I guess). My kids notcied this 1st , so here they are. Also i included shots of the other neons to compare. Plz keep in mind this is on a iphone in a planted tank...not eazy to get shots of, even if it isn't fully planted over.
this is one them.
Here r the other ones.

Any ideas?