Nothing about that sounds unique from existing CAPTCHA systems, nor does it sound hard to break. You're going to use CSS or Javascript to "hide" it from a computer? That uhm, doesn't make much sense (to me).
Not to mention something like Sikkuli (in JPython) can actually make your mouse hover and record values...
Plus how is that easy for humans to solve?
1. They need to know they can put their mouse over the text
2. Then record the text
3. Sum the numbers
4. Write the result...
If I had to do this every time I wanted to login to a site I'd seriously consider switching. And this is the 'best' captcha impossible to break - for humans. Machines can solve it easily.
