Anything is possible, Alyssa. I guess mishandled sphagnum could become contaminated somewhere along the way. I don't know why a staghorn would ever be potted up in sphagnum moss though. I do use sphagnum as a backing when I mount staghorns, and I have used it without problems in hundreds of mounts over several decades. Personally, once a staghorn is about 12 mo. old, I don't want mine growing in a pot of any kind, regardless of the media. That's just my opinion and preference. I mount all of mine when they are around 6 mo. old (that's age after I receive the seedling).
You can remove any contaminated fronds and spray a fungicide on the plant to easy eliminate the problem.
