I don't know anything about the plants but had read this and don't know if its the brown spots your seeing.
at:
Staghorn Fern, Platycerium
NOTE: Don't think your stag is sick or has a disease when you see brown fuzzy patches appear on the bottom side of the tips of older larger fertile fronds. These are the spores. Leave them alone :-)
http://mgonline.com/staghorn03.jpg