Sounds like it could be mildew or another leaf fungus, but I would need to see a photo to be sure. Better air circulation and more sunshine will help, so move the plant outdoors if it is a houseplant, or transplant it to a dry spot under the eaves of a house and out of the rain if it is a hardy hibiscus you grow outdoors. Good clean up of fallen leaves, and a mulch, rock, or wood chips at the base of the plant will help to keep this fungus from spreading. You may want to consider using a fungicide spray if the problem persists. Keep growing, Marianne Binetti