There is an octagonal pavilion on the opposite hill, half a kilometer from the balcony. Is it in Fengshui Mountain? thank you

Ah, there's nothing wrong with that. Generally speaking, there is the possibility of "rushing" in Feng Shui, but that means pointing to the door (for modular buildings, the door is a unit door, not a door of every household). Facing the balcony, there is no problem as long as it is not huge and has the potential to overwhelm Mount Tai. More importantly, the pavilion is a symbol of feng shui, which means "enjoying the cool", "calming the nerves" and eliminating disasters. You see, in the past, pavilions were often built in the yards of officials and dignitaries, that's what it means. Good luck.