That's the separation line between the cleaner and the protective layer of water. There's not much to do except change buckets occasionally, but be sure to use a steel bucket, not plastic. The cleaner will attack the plastic in a very short time. Use a LOT of high-pressure air through all the drilled passages after the soak and hot-water rinse to dislodge the gunk the cleaner softened. The carburetion instructor at the GM training center in Memphis used to say "Put 55 gallons of air through every hole in the carb when you rebuild it!" The cleaner just softens the crud- - - - -the air blows it out of the holes.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!