I’ve noticed a little bit of mold on top of the corks of some of my wines. Is the wine spoiled? Is the bottle leaking?

This isn’t necessarily a sign that your wine has leaked through. It could be that a small amount of wine stayed on top of the cork at bottling and has molded there. Carefully wipe the top of the cork and the bottle neck with a clean damp cloth before extracting the cork, and the wine should be fine.