It will be whatever gunk keeps the sponge soft in the pack. I dont know what is used, but it must be non-toxic and hygroscopic, so it wouldn't surprise me to find a salt/glycerine mix or similar which certainly wouldn't do your tip any good.
With the tips for your iron being the price they are, using kitchen sponge obviously isn't a smart move except in an emergency. Way back when, my boss had problems with unwettable Weller TCP tips. I never had that issue with my own tips, but I always flux and carefully tin a brand new tip before use and wash out new sponges etc. but he was a jump right in and get soldering kind of guy so probably didn't bother washing the sponge.