Letitride3c

Did it bubbled? Became colored liquid? Vaporized? Didn't read/sound like it was a mix of 2 or more "unknown" substance for you to alert the FA, out of an abundance of precaution protocol ...

That could be someone mixing their own "Crystal Light" - a low-calorie powdered beverage mix that offers naturally and artificially sweetened beverage options in multiple flavors - has 0 calories, similar to what cruise lines are offering to all pax on select ships fleetwide, as "flavored water" instead of filtered, tap water made from. CL is available in single serving packets, good for a 16 oz. sports/water bottle or in bulk container. We take a few along & threw it into the packing cube's zip-lock bag, and, TSA-compliant, thus far ...

It could've been a health or holistic supplement as already mentioned, or maybe something like Bayer Quick Release Crystals - 850 mg of aspirin and 65 mg of caffeine per dose - pour into bottle, shake & mix well, and consume.

The last time, on a long & delayed transcon. non-stop, I asked the FA to help fill a 28 cents worth serving of cup-a-noodles (from Walmart) with hot water from the gallery - we noticed a few surprised & "uncomfortable" looks around us, ha ha - LOL.