Currently Windows Viewer have 2 types of mouse control functionality:
Mouse local - works like a touch that shows absolute input position of the mouse pointer within your viewer screen. The mouse pointer changes to cross is a feature that indicates your mouse pointer is currently over the viewer screen.
Mouse remote - remotely control the mouse input and position across all the displays of your primary machine.
For now, there's no option to disable the mouse functionality, you just have to choose which one of these mouse functionality works for you or just don't click over the viewer screen to prevent the mouse functionality activation.
You can set in the Windows Viewer's Functionality settings before you connect.