We might get an answer in the third season. Or maybe not. Though I was surprised when they brought back Chris, so it may just take a while.
The fact that Levi just disappeared and that convenient new merman just randomly showed up to take his place in the plot feels like a hasty rewrite to paper over something happening with the actor, given the lack of any further follow-through. They might explain it away after the fact somehow, but it feels unplanned.