It's chipped, not acid eaten. You've hit the edge of the ram with something and it's chipped that finger, same as if it was the paint on the edge of a car door. But thats a considerably large chip to have flaked off, so could possibly be a manufacture defect if the finger naturally seperated from the pcb just by socket insertion.
If you look closely at those fingers, you'll see a darker line in the center. That's where it's actually making contact with the socket pins. That 1 finger is up beyond the contact area, so totally useless.
No idea if that's a power, ground, input or output data key or if that'd fully or realistically affect the ram overall. You'd have to memtest the ram stick individually, with comparisons against another...