My guess is, it's one of the cyanide variants that's made the rounds in recent years. I've seen similar videos in the past. Someone sticks their hands into a clear solution to show how "safe" it is. Then they usually dunk some gold plated fingers or other plated components into it, and the gold magically disappears.
Done as they show, it is relatively safe. A cyanide solution is relatively safe to intact skin. That changes if there are any scratches or cuts on the skin. As others have pointed out, it changes even more once metals are dissolved in the solution, especially very hazardous metals like nickel, copper, the PGMs, etc.
Dave