Warmth, darkness, moisture, and nutrients favor the growth of mold. I wish I could see your feeder to see how it is made. I'm assuming that your feeder has a glass container that holds the sugar water. A rubber cork fits into a hole in the glass container, and a glass or plastic tube fits into a hole in the center of the cork. At the end of the tube, where the hummers drink, there is a separate red piece that fits on the end. I'm assuming that it's on that red piece that you're getting the mold?

Do you normally clean the feeder every few days with vinegar or bleach and yet still get the mold? If the mold problem seems really tough, in spite of constant cleaning, I'm thinking the problem could be that you have a detachable piece on the end of the tube, and moisture is able to collect in the crevices between it and the tube. This could be encouraging the mold. I wonder if there's some way you could just take the piece off. Is it detachable? If you need the end to be red, though, maybe you could find a substance to dip the end into--some red paint or coating that would be non-toxic and would stand up to being outdoors. I don't know what that would be, but getting rid of a detachable piece might eliminate the cracks and crevices and there would be no place for the mold to adhere.

I don't know if this is your problem or not. You'd have to explain more, or post a really good photo. Or maybe someone else has a better idea other than frequent cleanings.