Sand type of wall/plaster

I live in middle east, dubai to be exact. Im trying to fix my wall from moisture damage. I scraped most of the peeling part of the paint and white plaster with water discoloration. But there was somewhat a 3rd layer which is like a sand in texture. I tried poke it and its about more than an inch thick before the concrete.
Now, id like to repair it so i bought a plaster and paint. Any tips on application? Because i am having doubt that the plaster i have wont stick on the sandy matrial on the wall