Diagnose My Plec Please

That looks very odd 

It's not whitespot, it is likely either fungus or a bacterial infection. Do you have a quarantine/hospital tank? If you do, I would treat it in there with a medication that treats both eg eSHa 2000

To be honest, I don't know what this is, I've never seen anything like it before.

I don't really like treating the whole tank - and therefore other healthy fish - unless there is no option. I would keep an eye on the plec for a day or two and see what happens. If it looks as though whatever it is is getting worse, then yes, treat the whole tank.

It has occurred to me since my last post - has the plec rubbed against something? They do like to squeeze into small places, could it have either caught itself and taken some scales off, or rubbed something off the bottom of a piece of decor?

To be honest the silver balls look like air bubbles to me, does it ever come to the surface or play in a stream of air bubbles from an air pump? That would explain why they've disappeared.