I know it’s annoying. Unfortunately, this has to he coded into the app, there’s no way to globally hide the mouse cursor. And it wouldn’t make sense, either, at least for the majority of Android apps. If you hide the cursor for the external mouse, you won’t see where you’re clicking (unless you’re running an emulator, but that’s probably 0,0001% of all users ?).
If you’re rooted, you can try to find the mouse cursor image in your system files, and replace it with an empty image. But that could brick your phone, and cause all sorts of issues. ?♂️