How do I get rid of this chalky white stuff?

Here is a website to help you. It looks like you have effervescents.

Hi Ruthie, that looks like efflorescence coming up through the stones. Is the ground wet underneath the patio?

this is a type of mold here are some solutions for you

https://www.radonseal.com/cleaners/RS-efflorescence-cleaner.htm

https://www.youtube.com/watch?v=vcESZOKTu1M

https://www.youtube.com/watch?v=koVSgTwp6kg

Have you tried white cleaning vinegar and a stiff brush?

It looks like a product was used on the stones because of the continuity from one stone to the next. I could believe effervescence if the pattern was more random.

Get a bucket of warm water and add 2 cups of vinegar. Scrub with a push broom and rinse.

It looks like salt scrub with a scrub brush use soap and water and a scrub brush it should come off then hose

Try diverting water away from patio. Seems it's getting too much moisture. Then I'd visit my local tile store to see if they have other ideas to stop the effervescence.

I think this is effervescence. Take a look at this video:

https://youtu.be/AEGLg8kT-Oo

Try 91% isopropyl alcohol with steel wool.

Hope this Helps! Jeremy - https://pahjodesigns.com

Hi Ruthie, I found this info for you and have experienced the same type of "white stuff" on concrete patios in the past. This is one of the most common but least understood phenomenons with concrete. Efflorescence is a chalky white salt residue that can occur with any product containing cement. As moisture migrates up to the surface of the concrete, it carries along with it calcium salts from within the concrete.

Murica acid. Use protective gear.

Hi Ruthie: Both efflorescence and calcium deposits can be cleaned using a 50/50 mix of white vinegar and water. Using a scrub brush, spot treat affected areas with the vinegar mix. The acidity in the vinegar helps remove the efflorescence and calcium deposits by breaking down mineral crystals. Sometimes simple washing can sometimes remove efflorescence. Scrub with a stiff brush and mild detergent, like Dawn and plain water. Good luck

Thank you so much! I’m going to try it!

If the efflorescence returns, liberally apply Wet n' Forget Outdoor cleaner, using a garden sprayer ( not a power sprayer). Do this on an overcast, windless day when no rain is predicted. The efflorescence will take time to disappear, but this does work. Reapply yearly, as needed.

This works well: https://amzn.to/2yYpPeS