Not a problem. All maples do this when the temperatures are switching between freeze and thaw. Some died starch gets converted back into sugar that gets dumped into the xylem lumens. A stem pressure develops by osmosis. So when you cut there's a little leakage of this sugary sap. Btw, this is where maples step comes from.
The worst that happens in your case is that the residue of this sugary sap becomes the substrate for some blackish mildew. All you need do is to wipe/wash it off.