Looks like this coin is either in a very late die state, has been in some acid, or both. Even though it has a shape similar to an S, that mark is almost certainly not a MM. Perhaps others will chime in about whether it is a die chip or something else.
Hi! I have an almost identical 1970 nickel - mine has some damage on the obverse though. Not sure if the holes are related to the odd texture or if they happened separately. I just found it today, and am completely mystified by it. Not sure if this makes the mystery more or less mysterious. --josephine
I don't think it's a coating, because it just feels like it's part of the coin. Also, the fact that mine and bryantallard's are both 1970 makes me think it's a mint thing.
At this time, I think it may be a weak strike. The details of the coin are not clear (even for a circulated coin). If the dies were spaced too far apart or if they were really worn (common for nickels of that period), then when the coin was struck, the metal would not flow well into the die details and into the fields of the coin itself.
both of these coins look like they may be metal detecting finds. when a coin has been buried in the ground and dug up after many years they are eat up with corrosion. when cleaned this is what they look like , acid will make them look this way also.