So, i’ve mounted tablet to wall, it has always open home assistant. Screen wakes up when motion is detected - motion detector android software.
Problem is that a TV is in the same room and tablet keeps turning on or off while watching a movie. Is it possible to add a button in home assistant to stop motion detector on a table until it’s turned back on?
That depends on which software you are using on the tablet. If the software has some API that allows you to switch of motion detection via REST or similar, then you can create an automation that does just that when you turn on your TV.
I am using android on the tablet. i think it could be done via mqtt client and tasker?
That’s what I thought. But which App? I mean the one you refer to as “motion detector android software”.
this one 
but i cant set the tasker if mqtt receives certain message 
As far as I know there’s only a Tasker-Plugin to publish MQTT messages, not for receiving. You may consider switching to WallPanel, which explicitly has the API to be controlled via REST and MQTT. In the current version though it also does not have the option do deactivate motion detection. But I’m sure the developer could implement that very quickly.
It’s funny because this mqtt client also receives messages, but it seems that tasker plugin can’t receive, only publish 
I will try with wallpanel app, thank you i hope its not hard to configure. im still a beginner
I have two tablets running the WallPanel app, and it’s working good for me. But as stated above, currently you can’t disable motion detection with that as well. I belive you can tweak the sensitivity, so maybe that’s enough for your tablet to ignore the TV.
How does face recognition work?
That’s the only feature that doesn’t work for me currently. But I expect that to be fixed. The app is still early in its development. But yes, using face detection could solve the problem as well.
i turned face recognition on and now the tablet’s screen wont turn on anymore?
update: i woke it up somehow
It’s all in early stages right now, so expect odd behaviour. 
Yeah in 0.6 the face detect is literally leveraging the code in Android to do face detection and my experience was it was pretty lousy. But the framework is in place to plug in all sorts of handlers. The QR code reader is probably of low use but as a proof of concept, it works a heck of a lot better
Another thought is if the MJPEG feeds well into OpenCV, HASS could handle a lot of things. Depends on if your HASS is better hardware though. But it will probably always be an option you could play with.