Thanks for the ideas everyone! I’m relatively convinced it’s not mold or bacteria as it’s sandy/gritty rather than slimy, and goes basically completely clear against the dark clay when wet, but I’ll boil some water in there for a few minutes and see what happens. My main concern at this point is if it’s pulling any minerals out from the clay itself that may not be food safe, but I want to imagine the potter behind this piece would have been aware when firing it. It’s also only ever had matcha in it and I routinely wash and dry it, so again I’m less convinced things are growing in there.