What is the difference between these two cursors, with tmux?
Normally when you for example select a piece of text in iTerm2, your mouse activity is reported to the application you're currently running (for example a text editor). That application decides what happens.
However, when you hold down the alt/option key in iTerm2, you disable mouse reporting. This means that the application is not told of your mouse movements, and instead everything takes place inside iTerm2.
This can be used if for example you want to select and copy a piece of text to the macOS clipboard, but the application you're running does not allow you to do so (i.e. it has some other behavior triggrered by the mouse).