Your definition is not really the best even for text:
\textfrc will be much safer.
For having it also in math mode, define
would give the expected result (which, in my opinion, is quite bad).
Note. Don't use
\textfrc in math mode if you load
amsmath, because it would try loading font sizes that aren't available.
There are several fonts that have cursive Es close to that in the sample.
(SIL license) http://www.fontspace.com/jean-douteau/ecolier