Corroding copper (the coil's tubing is likely copper) produces a compound that color. You should check the drain pipe and the drain pan under the coil and make sure it does drain properly. It looks like the drain is plugged, overflowed, and that's the reason for the staining on top of the furnace. Long term, that could rust out things inside, decreasing the life of the thing. Also, if the drain line does not have a trap in it, one should be added. I have one similar to this...it makes it easy to clean things out with the supplied (on some versions) brush.