Your problem is that the tile-store guy was an idiot for selling
you mastic to install such large tiles, and the HD person was
an idiot for for selling you that "membrane". But don't feel
bad...it happens everywhere- everyday.
Ideally you should have used cementboard or hardibacker board,
but "greenboard" or otherwise called MR drywall should be OK for
many years. With a large tile over 8x8, you should have used
multi-purpose thinset. "Greenboard", or MR drywall, has a
moisture resistant paper on the surface, which is what you were
trying to achieve with the membrane. I've tore out hundreds of
rotted tubs and showers made with it, so it isn't a first choice.
Membrane or not, the mastic needs to air dry and cannot.
It has probably dried around the perimeter of the tiles enough
to hold them, and if no water gets behind them, they probably
would never fall off. Suction will hold them, but if moisture ever
gets back there the mastic will turn to mush, so I hope you did
a solid grout job.
The mystery is this "membrane", and whether mastic will stick
to it. Multi-purpose mastic is pretty sticky stuff, and probably
you will be OK. I wouldn't take it apart at this point. Get a
box fan or something and keep a fan on it for several weeks.
My guess is by knocking on it, you can tell where it is bonded
and where it is not. For now the moisture will have to escape
out through the joints, so having a fan on, or A/C on, will dry
out the bathroom air and draw the moisture out.
But......it will take a long time.