It's an old leaf. Those kinds of leaves almost always die back first.
I don't know how often you water, but it could use a bit more water.
Btw, if you want your Miltassia to look nicer, I highly recommend pointing the new growths towards the light source. You don't have to, but it's just a little aesthetic tip.
Since your Miltassia is an intergeneric hybrid of the natural genus
Miltonia and
Brassia, the proper way to write your orchid's name is
Miltassia
Shelob to denote that it is a hybrid. In fact, a few people might get up in arms and say, "Why are you being the orchid name police?", either seriously or in a half joking manner. Right now, it's easy to just dismiss this taxonomy rule because it's pretty clear to people that the name is the name of a hybrid. Wait until you start finding hybrids with species looking names and I think you're likely to gonna want to obey those taxonomy rules by correctly writing those names out. It's going to be very difficult, (or rather, I should say, extremely confusing), to get the proper information down if people throw these rules out the window.