What kind of material is this (for a shower pan) and how to remove it

That looks like a mortar bed with Turkey grit instead of larger aggregate. If you are removing the shower it can be busted up. Put a bar under one edge and a smack with a heavy hammer 3lbs or better and it will usually break (hitting over the void made by lifting) If you hit it while it is supported underneath it will only chip the surface most of the time lift it an inch or 2 and a heavy hammer can break it up. If you don’t lift it and just pound with a sludge hammer you can blow out the decking below. You can score it with a masonry grinding disk but this is really dusty.