This is very often seen with neons nowadays. Earlier thought it to be a bacteria, but it appears to be untreatable and non-contagious to other species (in most cases). After a lot of threads about exactly this i've come to the conclusion this often must be Lymphocystis, caused by a virus (Irido) that's only found at neons and cardinals (probably Black Widows). It's untreatable, but can disappear on it''s own. Only lethal when organs are infected. White mouthulcers are common all over the world (Holland. UK, America, NZ aso). Must have to do with pressurebreeding in Asia (as Dwarf Gourami Disease = another Iridovirus).
Cheers Aad