ᐅ Renovation of Flush Base Slab/Foundation Slab

Hello, one corner of our base plaster is peeling off because it is damp. There is no moisture inside the house. Lava flows and lava gravel have been placed next to the house. The plaster has a black layer, but it only goes up to the upper edge. Last year, I tried to repair it with renovation plaster, but I probably didn’t remove enough of the old layer. How should I proceed?
Thank you!