For a San Francisco Buff, 1930-S usually isn't too badly struck. Your coin looks to be an average are slightly better strike for the date and mint. The usual die clash has been polished out (notice the weak right rear leg on the buff, weak lower second feather, and loss of detail at the throat).
In terms of grade, your coin has a full horn, but the tail has merged with the buffalo's hindquarter. That locks your coin into either a VF-35 or EF-40. On your coin, barely half of the hair is visible on the buffalo's head and on the buffalo's hump. That's the indicator for EF-40, though it is a close call here.
The corrosion spot is troubling. Most likely, that's copper carbonate, from a reaction to copper in the alloy. Maybe worth a Verdi-Care treatment?
Would you be willing to post high resolution photos of LIBERTY and the mint mark? There is a subtle
DDO for 1930-S and a couple really pesky and hard-to-find RPMs for this date. Any of those could balance out your purchase.
Totally useless trivia time. Your coin's obverse 7 (ribbon behind the last digit of the date, used from 1926 to 1937 except 1931 and 1935) is the most common of the 9 obverse designs by mintage. That design was suspended intentionally for the next year (1931-S) because of concerns that the ribbon would merge too quickly with the last "1" of that date, with fairly little wear.
Thanks for sharing your coin with us.