Renovation of flush base floor slab

Hello, a corner of the base plaster is flaking off because it is damp. There is no moisture inside the house. Lava flows and lava gravel were laid next to the house. The plaster is covered with a black layer, but it only goes up to the top edge. Last year I tried to repair it with renovation plaster, but probably did not remove enough. How should I proceed? Thank you!