My Pheidole pilifera colony. Not that you can tell what they are though from this photo. These ants are incredibly small and normally nest in sand. This is the whole colony and they could all easily fit on the tip of my thumb. They're not digging into the plaster but are able to nibble at it. They do this mostly as a reaction to water, or in this case condensation that sometimes forms on the side. They probably won't dig tunnels into the plaster until they've expanded into the other chambers. That's a long ways off though.
They're in the middle chamber to the far right. The Test Tube is no longer in there of course, this is an old pic from the summer. I keep the surface area clean as best as I can. Black patches of mold grow if food is left there. It scrapes off easily though. I don't think I'm damaging the plaster at all. I'm really surprised that that's the only place where mold is growing. The whole setup is basically a sponge and only the parts with open access to air are really growing. Plaster setups really only last 2 years anyhow and it will be easy enough to move the colony.