moss killer?

[Smallfry]

I would use a glyphosate weedkiller on it. It is systematic, so NOT instant gratification, and will kill most other weeds too.

Sulphate of iron works well, but stains like rust, and smells like it too.

Anything "sodium" is not good for concrete in the long term.

 

[Popcorn]

SODA Crystals spread them about evenly over the moss, the water it. Next day all the moss is dead shovel it up.