Why teacher's pet. he showed more and had more energy than those criticizing him
Absolutely.
It's such a bizarre thing to use to attack a player and, if true, reflects terribly on anyone who would criticize Elanga for it. Especially because you know they're only doing it because he's a young player and they feel they can get away with it.
Now, obviously, Elanga can't be mentioned in the same breath as Eric Cantona, but it's well known that when he arrived at the club, he'd routinely do extra training - and instead of calling him teacher's pet (which, to be fair, I would love to have seen as I imagine Eric's and SAF's responses would have been brilliant), the others got on board and worked harder to match Eric's standards.
Just because Elanga's young and maybe technically less gifted than a number of other players, that doesn't mean he shouldn't be acknowledged, praised and - ideally - emulated for doing what's within his own power to improve and impress.