You have taken the old adhesive & final finish plaster coat off which is sometimes inevitable I'm afraid. What is visible there is the browning coat. This will need to be cleaned up and a new skim of bonding/ multi finish over the top. Bonding isn't really suitable to be tiled onto yet some do. Some manufacturers say it can be if properly prepared but better off using multi finish.
Or take it all back to brick/ block and dry line it would be my choice if being tiled