What is this vitiligo-like disease called?

A few month earlier, I bought a pencilfish that had one pale scale in its body, over time that whitness grew and now the whole belly and half the back of the fish is bleeched, no signs of illness or behavioral change, adjacent fins also became infected and lost melanin butt are healthy otherwise, can anybody determine this progressive problem?

What sort of fish is it?

There are no fish diseases I know of that look like that. The scales appear to be changing colour and that could be a natural mutation.

If the fish is eating well and acting normally, I would monitor it and take pictures every week (or more often) to record what is going on and see what happens.

It's possible that the fish has something going a bit wrong with the processes by which colouration is expressed. Many species of fish can change colour quite markedly in a short space of time, and some species of pencilfish exhibit different colouration at night time v daytime.

As Colin says, if all else appears well, then maybe not something to worry about too much.