When the input bar is disabled, the smiley at the left is still drawn on a white background
Whenever you aren’t joined in a MUC, or even connected to the server, the input bar is disabled to prevent you from typing a message you won’t be able to send. This is visually displayed in a different colour, in my theme (default Adwaita) a light grey. The background of the smiley button in the left part of the input bar isn’t respecting that colour, instead it stays white during that time.