Is this a fungal infection on masdevallia mendozae?

[RandomGemini]

Not sure what to do... I love this little plant. I have had it in my terrarium, but when I noticed that one of the tips of the leaves had turned black and there were some dark spots on the backs of other leaves and a couple of leaves turning yellow, I figured it was time to pull it from the terrarium and find out what's going on with it.

If it's a fungal infection, what do I treat it with and how?

It's such a tiny little guy, I'm afraid to get too aggressive, but I like it a lot, so I don't want to lose it to this either. It's potted in a 2 inch clay pot in pure NZ sphagnum moss.