Anatomy of a concrete wall – what are all these layers called?
Yep, you got the Plaster right, it's that coin-thick smooth white finish coating. But, that gray nook &amp; cranny stuff looks to be a Cinder Cement, this stuff is quite weak compared to concrete.

If you were scraping off big bulges &amp; partially loose exploded chunks of the gray stuff, then it's definitely Cinder Cement. That's exactly how that stuff reacts to moderate water exposure &amp; freezing. It's not Stucco nor old plaster work's Brown or Scratch Coat.

Just fill in the big voids with most any kind of cement &amp; then re-Plaster over minor cracks &amp; holes while skim coating the whole room in Plaster. If this will be painted, then the Plaster can just be a Drywall Joint Compound. This is the cheapest &amp; plenty durable enough.