Will taking the math GRE subject test be a plus point for admission in MS/PhD in computer science?
It would be easy enough to ask any department to which you are considering applying whether they would give consideration to math subject exam GRE scores.  They shouldn't penalize you for asking (and at any rate it would be worse to send the scores without asking first).

However, I am inclined to agree with @aeismail: the fact that they formerly had GRE subject exams in CS and no longer do must say something about what at least many of the departments are looking for. 

Perhaps I should mention that I was involved in making the math subject GRE exam a required part of the application to the PhD program in my department...but my department is a mathematics department.  Still, after four years of doing graduate admissions I have some experience that seems relevant: in doing admissions one really wants to "compare apples to apples" whenever possible.  For instance this meant getting everyone's understanding that after making the math subject exam a requirement we really would seriously downgrade any application that didn't include it.  The reason for this is that when the exam is optional, a lot of students take it but don't send the scores if they are not strong enough.  This is obviously unfair to the student who turns in a lower percentile score: they are being penalized for their honesty and for following the rules.  I think similar considerations apply in the other direction: presumably very few or no other applicants to CS PhD programs are submitting this score.  So if you turn in a pretty good math subject exam score, what is the committee supposed to make of it?  How do they compare it with everyone else's application?

Considerations like the above make me think it is unlikely that you will be told that your scores will be considered as part of the application.  But there is no need to take my word for it: ask them, see what they say, and if you can, come back and tell us about it.