No rec letters from subject area of master's application
Oh, it doesn't really matter.
If it is course-based CS master, not researched-based CS master, then a recommendation letter only takes a very very small portion of it, so whether it is from a math professor or a CS professor doesn't differ too much.
GPA is the most important.
One of my friends who worked in the admission process before told me that the first admission step is to filter out students whose GPA is lower than 3.5 no matter which university they were in.