TromboneChat

Hello all! I’ve noticed these white bubbles on my handslide after bathing the instrument. Does anyone have any idea what it is or what solutions there are? Thank you!

Probably soap bubbles.
How do you clean your slides before bathing?
How do you rinse after bathing - and how thoroughly?
Do you dry your slides, inside & out after bathing? How?

Clean slide before bathing with a wide fabric strip (bedsheet, cheesecloth, ...) wrapped around a cleaning rod. And/or use a Slide-O-Mix cleaning rod with the appropriate towel sheath (red for small-bore, blue for large-bore).
[Be careful not to damage or dislodge the leadpipe inside the upper inner slide!]
Repeat after bathing to dry.

Outers or inners? You may have removed some lacquer if it's the outers.

Too-hot water can indeed remove some lacquer. Only bathe at a comfortable (for you and your hands) temperature.
The classic detergent for trombone bathing seems to be Dawn liquid. It works, but should be thoroughly rinsed.

Photos??

I've found some coatings can develop white spots when wet. This is due to the coating hydrolyzing due to the water. Often hot water makes it worse. Some coatings will then lose the white spots as they dry out. If this is what your coating does, don't panic. But consider cooler water temperatures for the bath.

If the spots are permanent, you have a more serious problem that may need refinishing.