hairline crack in gunite shell, patched with hydraulic cement

PB chiseled the crack and patched it with hydraulic cement (Hc), but the HC don't look very hard. does pebbletech will adhere to the HC?

A high quality cement, like Thoro Waterplug should be fine, though you may need to acid etch it. If it's Lowes' special "Chinese - We make cheap junk" cement . . . all bets are off.

But really, it's a question for your finish contractor. If they are any good, they will know what they can bond to successfully.