The correct expression is “en ce qui concerne”. The other expression doesn’t exist.
Saying “en se qui concerne”, would mean the verb is “se concerner”, but it doesn’t exist in French.

The correct form is “en ce qui concerne”.

“Se” is a pronoun (of the 3rd person), and is only ever used with pronominal verbs (do something (to) oneself – se promener, se laver, etc.). Aside from these, all grammatical constructions use “ce”.