ms is usually apply whenever someone types "ms excel" or "ms sql server" or anything similar into the tag field. Even its tag wiki says that it's pretty useless:
This tag is usually used to denote Microsoft, but is usually useless, please don't use it.
I've looked through the first few pages of questions and almost all seem to have a second tag that is more useful anyway, so simply removing it would have very little or no ill effect.
