My left ball of foot got numb when I walked with flip flops, what can I do other than stop wearing them?

Unfortunately flip-flops are one of the worst things you can wear to walk. Maybe get a pair of good comfortable sandals for walking.

There is specialized flip flops if its too hot to wear other shoes but if they are too expensive for try buying some sneakers that breathe so it's like having a air conditioner in your foot. If your foot starts to cramp up, you probably have planterfactitas.