At first I thought this might have been a parking lot coin, then a planchet or strike-through error.
Then you posted this picture.
(Sorry in advance, I like to over explain things.... a lot.
It definitely got splashed by something.
At the edges of the mark, whatever it was, slowly evaporated off the coin because of that cloudy color.
You can also see it on the back of the head. It looks like ripples in a puddle slowly leaving deeper marks as it dries, going towards the middle of the damage.
Swimming pool water with too much chlorine for example, would evaporate too fast to leave the deep marks on the coin, so it must have been something much stronger (possibly Nitric or HCl), or it would show a lot more environmental damage from sitting there for a very long time.
Even low PH "acid rain" could react in the same way eventually. There's plenty of ways to dissolve the metal in coins, lemon juice, vinegar, soda/pop, etc.
As for the letters being intact, they look pretty messed up also, like they've been melted a bit. An acid wouldn't go straight for the letters first thing and wipe them completely off the coin. It would slowly react to all of the metal that it was splashed on.
The lettering has a smaller surface so it would dry faster off the top and could take less damage that way. Plus gravity would pull the liquid off the sides to a lower, or more flat surface.
The acid would react to the individual atoms of the metal, which may have been slightly clumped together more in certain areas, or had microscopic air pockets and was able to get in there and dissolve away that part a little quicker.
It does look pretty neat though.