I have this code,
poke(0x5f2d, 1)
mset(oldposx,oldposy,oldtile)
oldposx= flr(stat(32)/32) + 8 + flr(player.camx / 3)
oldposy= flr(stat(33)/10,10) + flr(player.camy)
oldtile= mget(oldposx,oldposy)
mset(oldposx,oldposy,1)

The tile doesn't change at the mouse's location.