In case if you connect in a Full Control connection mode, then the tech behind the Viewer has control over the mouse. However, the remote user at the Host machine still can control the mouse pointer if the tech at the Viewer's side is not moving the cursor.
Please note that you can lock the remote user's mouse and keyboard when connecting to a remote Host machine. In order to do so, please enable the Lock host's mouse and keyboard checkbox in the Connection Properties - Additional properties tab.
Alternatively, you can also connect to a remote Host via the View Only connection mode - this connection mode allows you to view the remote screen without access to the remote mouse pointer and keyboard.
If you are connecting to a remote Host via the Full Control mode, then this behavior is normal. The Viewer's cursor always takes precedence over the remote cursor, so if the mouse on the Viewer's side moves even slightly the remote cursor will return to the current position of the local cursor regardless of where the remote cursors move. However, if you want to let the remote user work without such interruptions you can switch to the View Only mode within the remote session by clicking the corresponding icon on the toolbar:
Alternatively, you can also use the Alt+F12 hotkey combination to quickly toggle between Full Control and View Only mode within a remote session.
I have found this to be true: I connect in Full Control mode; even when I, the viewer, is not moving the mouse and do not have my hand on the mouse, the host has no mouse control. I recall that this was not the case in the recent past - I could simply stop using the mouse and the host could then use the mouse.
The Host side user won't be able to control their mouse only if you enable Lock host's mouse and keyboard in connection properties as Polina mentioned above.
If you did NOT enable that feature, the Host mouse should be available to remote user. Please, note that during a remote session the remote user will be able to move their mouse but once you touch yours the mouse cursor will "jump" to the position of your cursor.