That shower usually replaces drywall in that general area. So any little tapping (water) on the fiberglass shower will transmit effectively into the wall cavity. There it can travel up, down, sideways and of course straight into the neighbor's area. Any remediation at this point would be a major remodel.
Having said that, I would assume a fire rated wall is in place, meaning both your side and their side likely has a single layer of 5/8" drywall. Still, remediation at this point would be a major remodel.