There was moldy caulk around the tub drain. I intended to take it out, clean the drain area, reapply any teflon tape, and re-caulk the area. When I started to turn the drain counterclockwise the top piece popped off.
It appears to be a drain cover. Here's the drain underneath.
We've cleaned the cover and the top of the drain... now I'm trying to figure out how to put this back on. I was going to apply some plumber's putty to the underside of the cover and stick it on... does that sound like enough adhesion? Should I re-add caulk around the cover once it's back down? Since the cover sits up higher than the drain below there's water that pools around the cover (hence the moldy caulk I was removing) so I'm leaning towards not adding more.