I get green moss (or mold) on my concrete in the back yard every winter. I power wash it and reseal it, but seems to come back every year. The area affected does not get sunlight in the winter months.

Any suggestions??

Not uncommon under the conditions you describe. Wait till it is warm again, soak with an algae/moss killer solution, power wash, wait until the concrete is very dry and then apply a concrete sealer.