What can I do to save the marble bath tub/wall?

what u sprayed on is unfortunately a calcium and lime remover. Your marble is calcium, so in fact you have removed a layer of marble. OR you removed the shine. The ugly film is burnt calcium or calcium residue. It feels like a coating but actually the marble has now been textured as a result of the reaction towards the product. Just imagine heat on plastic, the way the heat would bubble the plastic. This is basically the same thing.

The only way to bring it back is to either

have a professional repolish the surface or replace the effected tiles. The residue in the bathtub can be removed by applying more of that product on top of it, but ONLY on the tub, not on the marble of course, the leaching residue will dissolve.

Marble is porous, whatever you sprayed on it likely is now in the marble.

Take the can to a tile store, NOT a home center, and ask them what you can use to remove the residue.

I'm not sure if there are DIY way of fixing it, Just try to visit http://www.bathroomandkitchenguide.com/ they have listings of bathroom companies that offers restoration services for bath tubs.