Seems I found the "sweet spot" because as I write this, the ghost is far off the right side of the playable area racking up score (4k+). The controls for ascend/descend work, but something forced the ghost to rush right and I can't seem to get it back.
I might suggest figuring out what made him flee, and maybe adding some stalactite obstacles which would end his freedom if someone else happens to find the no effort sweet spot (13k+ after proofread).