Water in the ultrasonication bath
To see these jelly flakes in an ultrasound bath is a normal thing -- referring to ``it happens regardless if the water was tap-water or deionized water, a just a matter of time''.  Perhaps this occurs faster in the former, than in the later, especially if the bath is used to clean glassware.  But it equally happens with glassware judged as clean (by eye) for example if the bath is deployed to initiate / support a reaction inside a reaction flask.  Ultrasound-assisted generation of Grignard reagents is one example.

The ultrasound bath works by sonication, and the cavitation simply detaches the oily/fatty/grease surface film covering quickly all what was touched once by naked hand (e.g. sweat), and breaks this into even smaller pieces.  Over time -- when the ultrasound bath is switched off --, this thermodynamically unstable emulsion decomposes, splitting into a water soluble phase, and a hydrophobic phase that then tends to agglomerate.  (I did not check the presence of organisms living in or on them since discharging the bath is a chore regularly done, too.)

Side note:  For me, these residues appear to be more a liquid, then a solid -- hence I used the term emulsion.  If they were a solid, the thermodynamically unstable mixture would be an suspension.