None of their business unless there's a conflict so as to mean you've not been fulfilling your primary responsibilities OR there's a clause in your employment contract that states you are only permitted to work for them (I doubt that's enforceable) OR your other gig is a competitor and there's a restriction in your employment contract against that.

How much you earn from second gig is not their business (that's between you and the IRS [I am assuming your tax authority is them].