Receiving ETH, BTC or any other altcoin by a mistake is pretty impossible because there are so many possible crypto addresses (billions of billions?). So he has to know your address and then send it to your wallet for example by copying wrong target address. And what I would do with it? Nothing, only if he contacts me for example by sending me a token with the name "Sendback" for example, I will return. But you never know why you receive the transaction and what his intentions are.