A pressure washer would be your best bet but if that not an option a regular hose plus bleach would be what I would try first.
I have used a spray bottle with a dilute bleach --- with good success.
IF IT REALLY IS MOLD ;
The best solution is to scrub and clean the affected areas with a mold-killing detergent or a simple solution of household bleach and water. Be sure to scrub aggressively to get down into the pores of the concrete. Let the detergent or bleach solution soak in for a while to ensure that all the mold is killed.
IF IT IS GREEN ALGAE:
Vinegar can also be used to rid of algae. Use a mixture of water and white vinegar to spray down the area and kill the algae. Rinse the surface alga off the concrete with a water hose. Next, pour white vinegar directly on the surface and scrub using a brush.
Windex also works very well on algae.
We use Simple Green and a power washer.
Use a bleach solution
This should help: https://www.environix.com/mold/learning/basement-mold/mold-on-concrete/
Bleach will clean the mold. A 50/50 mix of bleach and water. Vinegar will kill mold spores. A 50/50 mix of vinegar and water. Do not mix the two. The fumes are harmful
White vinegar with a bit a of blue dawn used with high pressure water or pressure wash.
Bleach and a pressure washer will work great at removing mold from concrete! Also works great at removing paint paints. :)
Use Spray and Leave. It will do the job for you in a few weeks............
Diluted bleach in a spray bottle. Spray and leave
Yes, with the concrete being porous, the mold or algae is in the pores, which means it may come back.
If you’re going to scrub a bleach solution, get a scrub brush on a stick, used for scrubbing wood decks.
Also keep in mind, that whatever clothes you’re wearing may get bleach splatters on them.
And run/off from the rinsing process can kill grass and many cities/counties prohibit toxic run/off into the curb gutters, as they drain into waterways where fish live.
How much mold is there? I'd try a magic eraser if it isn't too bad.
White vinegar kills mold, just apply it with a spray bottle.
Where is the concrete, indoors or out? That will make a difference. If indoors, you’ll have to fix the water issue first.
Here's a post that might help - https://www.concretenetwork.com/fix-concrete-cleaning/clean-mold-on-concrete.html
On an overcast day when no rain is predicted in the next 5 hours, spray the concrete with Wet n' Forget, using a garden sprayer (not a power sprayer). Protect any nearby vegetation by first wetting it down before applying Wet n' Forget. Over time, the sun and rains will eliminate the mold.
This stuff is also excellent on vinyl siding and pavers.