The jellyfish isn't privy to Wash's thoughts, but it seems to know that this is a moment for quiet and stillness. It stays in place, beaming the same few seconds over and over in a loop, until Wash turns back to it to say goodbye.

Weirdly, it seems to know something nice is happening when it's thanked, too. Some excited bobbing happens, and it drifts in a circle. Then it reflects Wash's smile in its bell. He's very welcome!