Yup, that sounds like scale. Aspleniums (including bird's nest ferns as well as various others) are very sensitive to it, and I haven't figured out any feasible way to keep them under control. Apart from toxicity problems, pesticides don't seem particularly effective. Picking them off by hand can work if you do if often and thoroughly. I don't know about staghorn ferns, but ficus certainly can get bad scale infestations. Also, for what it's worth, the sori (clusterings of spore-producing sporangia) on a bird's nest fern would look like straight dark lines starting near the center of the leaf and extending about halfway to the leaf margin. Here's a picture I took of the underside of a leaf of a large bird's nest fern with sori: Patrick Alexander

I have had good luck killing scale with 'simple green' soap. Got desperate one day and tried it just spray it on them. I think it melts the prtective shell.

I have the variety "Jack Frost" which is similar. The first leaves in the spring when the plant flowers are smaller. After, the plant develops significantly larger leaves that vary somewhat in appearance. So I'm thinking that the plant you have with the larger leaves is just further along then the other, because you have it in a sunnier location. Both are lovely by the way.

Pics aren't clear enough, but I see oedema and possibly mechanical injury (or more oedema) on the leaf margins in the second 2 pictures. Can't tell what the dark spot is in the top picture, but if it's scale, it's easily removed with a fingernail, wooden skewer, or similar tool. If it's scale, it won't leave a 'crater'. Al