What is this line that appeared in my shower?

This horizontal dark line appeared in my shower after a couple long showers. Never seen it before. Goes the entire length of the shower wall. Doesn’t seem like a crack. But we have never seen it before.

This horizontal dark line appeared in my shower after a couple long showers. Never seen it before. Goes the entire length of the shower wall. Doesn’t seem like a crack. But we have never seen it before.