I started to notice it on the bottom of my bong after I washed it with salt about 6 months ago. That was the only time I ever used salt to wash it though. I only use dishsoap now. I usually clean it before i use it and put it away back in my bag but i usually put it away damp so I'm really hoping its not some sort of white mold as ive been smoking out of this for a while. i havent noticed any lung problems or health effects so yeah. what is this stuff? edit: this was the best picture i could get sorry. and its sideways if that confuses anyone. its not fuzzy or anything either. it just looks like white resin or something. edit2: the white lines on the side are just the design of the bong, im talking about the stuff at the very bottom of the bong.
Likely hard water stains, clean with iso and distilled water to avoid this. If you piece stays dirty and often has water sitting it in it could be mold but chances of that are low.
91% isopropyl alcohol and just regular table salt, pour the salt in first then cover it with your alcohol then cover all openings on the bong and shake vigorously for about a minute or 2,should cone right out. Works just as good if not better than the solution that they sell at head shops