Personally I will keep referring to it as stained clay. Although the term makes no real sense. Like come on, it's not stained, I deliberately painted it that way!
But whenever someone says terracotta, I only think of the new (glazed) ones and not stained clay. So that will be the bigger problem when communicating with other players.
I'm a redstoner and a modder.
Check out my youtube: grizeldi's youtube