@andreiniculae it's graphite. It helps with heat dissipation from the battery. If you remove it, nothing much would happen, maybe the phone's battery will get a little hotter but it will still work fine. Dont worry about it
You have to replace the whole screen. Since ifixit doesnt have a guide, you can search on youtube "screen replacement for samsung a30" . Hope this helps
I had this problem as well with an old hp pavilion g6. After I put an ssd and two sticks of 4gb ram, the problem got solved. If this does not work then maybe you need a better cpu as well. Hope this helps