Do we have this symbol in mathematics? 
It is the symbol I used to show left hand side of inequality isn't greater than right hand side.
More precisely I used this to show the below matrix is not diagonally dominant:
$$\begin{bmatrix}5&6&7\\
2&-4&2\\
3&2&-5\\
\end{bmatrix}$$
So at the first row I want to emphasize that $|5|$ is not greater than $|6|+|7|$  therefor instead of writing $|5|<|6|+|7|$ I used the symbol in the image above.
Is it ok to use this symbol in mathematics?

To me, it is OK. Putting a bar/cross on a pre-existing maths symbol just means the negation of it.
I'm not sure it is standardized, though.
Edit
As shown by @Dietrich Burde in his answer, these symbols are $\LaTeX$ symbols, so that's clearly OK you can use them.

