Wait several minutes and use a large scrapper to remove the mastic.
How to remove tile adhesive from plaster walls.
Removing tile adhesive from plaster walls.
Irrespective of the adhesive formula you have to cover the floor with old cloths as to protect it from the residues which will fall off the wall.
If the adhesive is not cement based you could try sprinkling water to the wall to ease the scrapping process.
I m hoping that someone can suggest an effective way to remove old style tile adhesive mastic i believe not thin set cement product used now from plaster walls.
Apply some water to a small section of the adhesive.
How to remove ceramic tile glue from a wall.
If the adhesive used was organic mastic the water will begin to soften it allowing you to.
Apply an adhesive removing paste to the wall.