This is a common sealer available at a hardware store. I have dipped the terra cotta tile and it has dried. The surface of the dipped portion is smoother and has a slight sheen. That is the price paid for sealing the matrix against freeze-thaw spalling.
Products like this are available at hardware stores. After you have removed the surface scum, be sure to seal it using a sealer (also available at hardware stores).
A common problem with dry and fired ceramic. It is evident by the presence of a light or dark colored scum on the dry or fired surface.
Outdoor Weather Resistant Ceramics
How can you be sure that the porosity of your fired ceramic ware is low enough to prevent freeze-thaw breakdown in the winter?