Hello!! New pool owner so trying to learn the ins and outs.

I just bought A set of steps and the previous owners did not clean it after they took their pool down last year. I have cleaned the outside and the inside of the railings where they had put the sand to hold it down. However, even after cleaning and spraying I am still getting what looks like black mold coming out of the step threads and risers. Any tricks with cleaning the insides of these things?