When the root ball has pretty much dried out and/or the fronds are drooping, that is a good time to water the staghorn fern. The grayish/ white substance underneath the leaves sounds like a fungus'or powdery mildew.
To be sure, cut a sample from the plant and bring it to your local gardening center for identification and treatment options. With that said, most staghorn ferns I see here in Central Florida have some brown fronds.
