How to insulate the outer wall against moisture?

Hello everyone

we have a wall in the outdoor area that used to be plastered. Due to years of weather exposure, the plaster eventually deteriorated and green algae developed.

I have now removed the old plaster and applied sealing slurry as a moisture barrier. After drying, you can see areas that are not drying properly. Is there another idea, especially to keep the base area permanently dry? I ultimately want to finish the wall with [Scheibenputz].

Thank you very much and regards
Tim