Placing tiles on concrete wall
There is almost no chance that is concrete.   That looks like some kind of concrete board material (if that) with mastic slapped on and tiles.   

Everything has to come down.

If it is concrete (don't think it is) then you need to assess the shower.   If the concrete is in good shape, remove any damaged concrete the put a skim coat of thinset, let dry.   Then tile with a generous amount of thinset (different question).   

If it is not concrete - after you assess when tile is removed... ask a new question and more than happy to help.   A concrete shower is awesome... unless you want it to be a different size!