No rabbitfish in the tank.
Livestock is typical cleanup crew and some small fish (chromis, clowns, gobies, a cardinal). I've had these fish for at least several months or years and I've seen most of them poop - it never looks like this. These are not like any fish poop I've seen before. I tried to break one up with a razor and I it was much denser/harder than I expected.
It's almost like pellet food, but after sitting in a tupperware of tank water since last night they're still hard and totally intact, they haven't broken down at all. I do feed NLS pellets but hadn't for maybe 12 hours before I found these, and at any rate the NLS pellets sink and break down fairly quickly.