This looks to me like mite damage. Do you have magnifying glass around that you can use for a closer inspection? I try to keep all fo the plants that I produce clean and bug-free, but using regular houseplants in a riparium can often introduce trouble.
This just looks like some shipping/handling trauma to me. Leather fern doesn't usually have problems with insect pests. I would just let that plant grow a couple weeks more, then cut off this damaged leaf.