What is the possible side product when mixing citric acid, NaCl, Cu, Al, H2O2, and an unknown photoresist?
1) You should not use acetic acid (or any organic acid) in this setup. It potentially may form some (even if minuscule) amount of chloracetic acid which is really hazardous. Furthermore, copper acetate (and other carboxilates) has low solubility. Go for either genuine $\ce{HCl}$ or for $\ce{H2SO4}$ used in lead accumulators.

2) Most likely it is aluminium hydroxide. If the substance is bluish, it may be also copper hydroxide.