From `man tmux':
By default, the window list shows the index, name and (if
any) flag of the windows present in the current session in ascending
numerical order. It may be customised with the window-status-format and
window-status-current-format options. The flag is one of the following
symbols appended to the window name:
* Denotes the current window.
- Marks the last window (previously selected).
# Window is monitored and activity has been detected.
! A bell has occurred in the window.
~ The window has been silent for the monitor-silence
Z The window's active pane is zoomed.
The # symbol relates to the monitor-activity window option. The window
name is printed in inverted colours if an alert (bell, activity or
silence) is present.
Since you didn't provide your
.tmux.conf, it is hard to tell what triggers the alert. Try
C-b : set-option -g monitor-activity off; set-option -g monitor-silence 0; set-option -g bell-action none and test if the problem persists. You could also forcefully disable the highlighting by adding
window-status-activity-style none and
window-status-bell-style none' to your.tmux.conf`.