Yankees pitcher Michael Pineda doesn't seem to be hiding the visible splotch of stuff on his throwing hand. It appears to be pine tar, as NESN color commentator Jerry Remy noted. (Remy wasn't happy.)
A number of MLB pitchers use similar substances—in addition to the legal rosin bag—and not everyone cares, but it gets attention when it's so blatantly obvious. Rays reliever Joel Peralta got ejected back in 2012 for a "foreign substance" on his hand and was later suspended for eight games. The Red Sox either haven't noticed or don't care, because no one's approached Pineda about it yet.