Found a friend in our terrarium. Must have arrived with the moss, or soil . . . What is it, and how can we keep him happy?

I shall call him "George" for the moment

This shelled creature is tinyyyy!
The first photo shows the size difference between my finger tips and George.

We put this terrarium together mid January, and included moss ordered from Etsy.

At some point we noticed George (for the very first time) sticking to the inside of the glass bowl. He moves around, and we lose sight of him —to spot him again not very long after. Tonight we found him on the bottom of the lid of the terrarium. It's not a great spot for him, but am worried that 'poking' him off, even gently, might hurt him. Hopefully he moves to a better spot on his own.

Our curiosity is piqued, we'd love to know what our new pet is — And how to keep him happy in his home.

We are planning to add springtails to the terrarium, next week. And I ordered 10 isopods [Orange Koi (Porcellio scaber)] along with the springtails — [[[But am rethinking the isopods, as I am coming to the understanding that the current terrarium is too small (with too shallow substrate) to accommodate them…. Anyways that's a question for r/isopods…]]]