Hi Thomas,
Welcome to the Paflyfish forums. The short, basic answer to your question is "no."
As the terms are used in fly fishing, the term "midge" implies a specific species of insect. "Nymph" means a life phase of many species of aquatic insects, including midges. So, for example, you could be fishing a "midge nymph" fly. Or you could be fishing a nymph fly that represents some other insect. Nymphs live underwater so virtually all nymph fishing is subsurface. If you're fishing a midge, however, you might fish it on the surface since adult midges are usually found on the surface. Flies fished on the surface are called "dry flies." Midge flies are fished both as dry flies or as nymphs but the nymph version is always fished below the surface, or just below the surface.
Also, midge flies are always very small, typically about the size of a mosquito. If you were fishing a "midge nymph" it would be very small, likely not more than a quarter inch in length. Nymphs of other insects can be quite large, up to an inch in length or even bigger.