Looks like some sort of oxidation that someone started scratching off of the obverse with their fingernail.
After running through coins with tape on them, grime, gum, etc. THIS coin looks like a gem compared to them!
That's the worst thing you could do for this coin, IMO. I've seen some really ugly toning on coins, and this doesn't even come close to the worst I've seen. I'd rate it as neutral in terms of eye appeal. I like that the coin exhibits originality from being in one of those old mint set holders. I'd be glad to have it in my collection if it were in any of my areas of focus.
I found this at an antique mall two days ago. I'm pretty sure it would qualify as ugly to most US collectors, but us collectors of ancients like dirty, crusty coins...
All I see is sweet, sweet originality. There's a possibility it'd benefit from an acetone bath, but she's beautiful the way she is. Great score, @John Anthony!
Actually as a circulated example that look is strongly preferred over the blast white look for most collectors.
Even the circulated ones? On a more serious note, a cautious numismatist suspects every coin before, say, 1950 or so which looks "new." Especially with silver, the majority of them have been dipped, cleaned or otherwise altered to, um, "renew" that look.