Two reasons this may have happened. One is your peanut butter had too much oil in it, to begin with. (see my photo above) Or you may have heated the mixture too fast and caused the oils in the chocolate to separate. To avoid this, I recommend heating in the microwave at 50% power and stopping before the chocolate morsels are completely melted. To fix an oily fudge, I recommend transferring the fudge to a clean bowl and letting it sit for a few minutes then stir the fudge slowly in order to try to incorporate the oil back into the fudge.