Clean out all the loose bits of rock and dust with a hard jet of water and encase what's left in a layer of mortar, as has been done to the top surface.

You have a choice of two options: Remove what's loose and use mortar to repair, or take the step out and replace.

a mortar skimcoat is basically just a band-aid/temporary fix. Unless that's all you can afford, I'd yank the whole thing.

Band-aid works for now. Going to be removing the front porch and rebuilding within a year or two.