On web, I use it on HTML.
But, Can UE4 recognize hex codes as character? Especially on UMG.
example:
 = face
Thats unicode code, byte value in unicode encoding that represents that character, you don’t need that code you can copy paste symbol anywhere where unicode is supported (not in C++ thru, you need to use this code in strings \uE87C). Unreal support unicode, you just need set up font that support specific character (again that code is bytes that represent character)
Now about your code, that code is in private range, in the past emojis was not officially supported in unicode, there for mobile phone OS developers used so called private use range (E000-EFFF) dedicated for your own symbols. So you should not use that code anymore as emoji been recently (Unicode version 6.0) added officially to unicode tables, you can check there codes here, use them instead (also in web!):
Ah i missed al ink so it custom font 
yes you can import those as a font one way or another under those codes they are designated. Read doc about font importing and font editor i linked
This específic font, respond with a “Face” if see “face” content characteres. Its Called Font Face, used in web for icons that respond for size of font. Instead make sprites (.pngs), we use fonts covered of symbols for icons, as trash, pencils…
Like this:
Or this:
I mean, can I use this on unreal?
Also, I´ve found the complete “icon x code” reference.
https://github.com/google/material-design-icons/blob/master/iconfont/codepoints
and converted here
But you wont be able to use , thats HTML thing, if you use C++ use \uE87C else where copy this copy paste it
Yes it does not, only in C++, elsewhere simply copy paste the character