Thinking about getting a masters degree in Mathematics.

I have a graduate degree in CS and have been working full time as a application / web developer.

Should I go for the mathematics degree???

Depends on what you want to achieve with it.
Will it Help you in getting a better Dev Job? Not really but it will give you Higher credibility when applying for Data analysis, ML positions and the likes of that
Do you have the Personal and financial freedom and love maths? Sure go for it

@don-rager
I really love mathematics and all of its application and I believe it will greatly help me if I prsue Data Science and ML in the future.

Do you want to get data science? Because that's how you get data science.

If you are asking us then NO.
You should be convinced yourself that you like heavy maths.

Maths on that level is HARD. If you really like it and don't mind how 99% of the curriculum doesn't have a use case then go for it, not for the degree but for the knowledge. Just don't expect to finish it. Lots of people don't.