what is wrong with my fish?

Sounds like he has a serious bacteria infection. The 'white moss' is more than likely a false fungus caused by the bacteria columnaris as true fungi infections are increibly rare. The 'bites' on his tail sounds like fin rot; an opportunistic bacterial infection.
I suggest gradually increasing his temp to about 82 degrees, add some fresh water aquarium salt and a mild antibiotic. The heat effectively induces a 'fever' but will also help kill the bacteria. The salt does this as well, preventing a secondary infection, and helps kill both types of fungi. There are many different meds available at your LFS, though I am a fan or triple sulfa, an API brand. Ive used it with great success on betta's with fin rot.
 
my guppy has this weird part on its body that looks like it is covered in white moss or bacteria. it also looks like that is a bunch of micro little human bites in its tail..whats wrong?!!?
 
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