New limp leaves and black spots

[jjohnsonvet]

Hello everyone,

I don't know much about orchids, but I have three now. I am concerned about one of them, a Mtssa. Izumi hybrid. I got the plant in August when it was in bloom, 2 huge spikes, and it has done very well. It has put on a lot of new growth in the past few month, new aerial roots.

I've placed it near east and south facing windows so it gets quite a lot of light. I water it once a week, and I mist the sphagnum moss that I placed around the base of the plant daily to increase humidity.

Recently, some of the leaves have begun to turn slightly yellowish and feel limp compared to the other leaves. There are also some small dark spots on the underside.

Can anybody tell me what this is? I don't know if its an infection or a problem with the environment.

Thanks!