It does kinda create the illusion of soapy bubble filled white water. If you change thinking about the ‘white water’ section to actually be the see through part in between clear round water droplets (the ‘bubbles’) that are clinging to the side of the container.
It’s most likely someone recently poured some water out of it from the spout at the bottom and what water is left on the sides from the former water line slowly makes its way down to the new water level.
It will likely scare them the first time as it’s new/unfamiliar, but they’ll begin to normalize it with repeated events (as long as there is no physical/situational aggressiveness toward them). Once they acclimate to the singing, they might still get a small jump scare if you begin abruptly belting out something, but as you continue the song they would likely relax back down recognizing it as not a danger. I find most pets will try to hide/runaway when they don’t like a sound, so indifference from a pet is akin to general acceptance of your presence imo.
Here’s wishing you a wonderful performance!