Phal NOID with Dry Black Spots Under Leaves

[blackvine]

Hi, all - I have a Phal NOID that has a fairly large (about 1 and 1/2" by 3/4") black spot under its lowest leaf. Right next to that is a grayish spot. Both are dry and neither shows on the upper part of the leaf. I noticed it about a week ago and it has not spread.

Background: I received the orchid about a year ago as a gift. It did well. It was in all sphag, but I just adjusted my watering to let it dry almost out, then soak. In June, it threw out a new leaf that is now bigger than the others. (A good sign.) In September, I thought it may be happier in CHC with a little sphag, so I repotted in 2/3 CHC, 1/3 sphag, and a half handful of charcoal. It has a new root as of about a week ago (most noticeable in the 2nd pic, at the base.)

Should I worry about this? I'm a bit new and not too worried, but . . . I may be wholly wrong!