Can you keep unfinished egg white in the fridge and use it again the next time you do a egg mask or will it not be good for the skin anymore since it is not fresh?

Yes you can, if you put it away immediately, do not eat it, put it near your mouth. But since there are so many products out there that do not have the chance of salmonella when used, I would recommend that you do that.

My basis for the YES answer is that it is not a spoiled product. You could egg yolks in a recipe, put the egg white into the fridge (in ie tupperware) to use up in approximately 5 days.

How about getting a carton of pasteurized eggs, such as EGG BEATERS. You could divide the carton into zip snack bags of how much you use, and then pop the rest into the freezer. Each would be fresh upon thaw. They are EGG WHITES that look like a whole beaten egg.

Good luck.