Galaxy Watch Active 2 back randomly fell off (they yellow gunk is the glue from the factory). What do I do? This problem...

Did you move files over to the new SSD? Did you have bitlocker on the old SSD?

I think this is a super common problem, if you look at a jerryrigeverything video he shows that for many modern phones they put a rubber gasket and also the microphone on an angle to mitigate this issue. You can try to call or test the mics to be certain, but you are safe