The mold is still there even though you washed it , spray it with
straight bleach or maybe 50 -50 water. Bleach will kill it but unless
you change the invironment there it will come back. I am in shade and
every few months spray with a garden sprayer bleach on concrete mold. Be
sure to rinse the sprayer bleach is hard on them. Paint , well you can
and there are additives, but the mold likes it there it can grow back.
Bleach is the easiest and quickest way to keep it clean