New SWG (IntelliChlor IC40) owner here.. had the same question about bubbles coming out of the returns when the SWG is running.
They're hydrogen bubbles. Hydrogen is a byproduct of the SWG process of converting Salt to Chlorine.
For some plumbing runs, the hydrogen doesn't get dispersed in the water and can form bubbles when it comes out of the return.
As long as you're not breathing the bubbles, you're Ok.
For a cool trick.. hold a lighter in the air just above the return. It'll ignite the bubbles as they reach the surface and make a POP sound.
Small, correction, the bubbles don't form as they're coming out of the return, but in the salt cell itself. Clear cells like Circupool's let you see the bubbles as it produces chlorine, regardless of whether they have time to collapse before leaving the PVC.
Kinda glad my bubbles collapse before my return, cuz I'd definitely try to set them on fire, lol.