No it didnt move like a slug (it head could “freely” move around like a leech) and over here we dont have such slim slug like these

Yup i think so, Microplana terrestris (given by someone else here) seems to match!

Yup, i think you are right! Still dont know how he managed to get there, but at least im a little bit less worried to take my shower now…

I had a lot of snails these days in my garden and these flatworm prey on them, so that makes sense.

I was almost sure too but i poured hot water on it to kill it (and kill any from my pipes), and the belly part is white (see pic) and i cant find any leech type with white “belly”.