Neon Tetra with white lump on mouth

HI there
One of my Tetras has a white lump on his mouth. He's not unhappy, is swimming around with his friends and eating well. He's had the lump for about a month, my fish shop said that it would go on its own but it hasn't. My new fish shop tested my water and gave me some Myxazin, which I've used for 4 days as directed but it hasn't made any difference.
My tank is 45 litres, with a couple of tetras, mosquito rasbora and some shrimp.
My water parameters (from the water test sheet) are:
PH 7 / gH 8 / kH 6 / Nitrate and Nitrite 0 / ammonia 0 / Fe is 10mg/l. Water values are optimal.

However, I have a snail problem (which my newly purchased Assassins seem to have no interest in helping with) and I've noticed some dark thread like algae attaching itself to my grasslike plants.

Any advice would be great.
Thanks
Debbie

 

Bumping this up for you

 

Has your tank been cycled? You should have some nitrates. Also, these are schooling fish and happier in a group. I'm not sure of the disease but this could be caused by stress.

 

How many tetras in total?

Could be fungal infection or stress like what the person above said

 

This is recently seen by a lot of neons all over the world. It doesn't seem treatable / curable.
That makes me suspect it is a viral infection like lymphocystis.
It doesn't seem to be very contagious or lethal.