Painted new plastered walls with matt emulsion without mist coat

Leave it. Let it dry, if after a couple weeks no peel is obvious you have been lucky. If peeling has started use a scraper to remove peel then knock up some filler and skim effected area using a caulking board or trowel. When enough time (this can vary depending on walls) has past without peeling spot in the skimmed area before coating up the whole wall.

what u can do b4 u paint if walls are over trowled when they were plastered u can bond them with pva bonding light mix of solution mixed with water and that will stop peeling but u need to be careful not to overbond as u wont have enough soakage for paint