I’ve seen videos that recommend protecting the finish on your uke with masking tape down by the bridge when changing strings.
Last time I changed the strings on my ukes that have slot bridges I worried about scratching them, especially the one that has a wound low G. It was hairy. But I was reluctant to use masking tape.
Is it safe? Will it leave a gluey residue? And if so, how to get it off?
I would completely avoid the traditional "tan colored" masking tape. Using it will guarantee remaining residue, especially if left on for over 15 minutes. (Many many manufacturers of this 'generic' stuff... you never know the quality of the sticker')
My go-to is Yellow FrogTape. It's considered the best for delicate surfaces (like painted surfaces that are holding tape while you are painting trim, etc.). I've left it on light colored latex painted surfaces for days and upon removing, no issues with tackiness or tearing off of the painted/laquered surfaces.
FrogTape also makes a green tape that's good for general surfaces and works as well, though I wouldn't leave it on as long, a few hours is enough. Take a look at the [ FrogTape.com ] website. BTW, your local Ace Hardware carries Frog. Local Aces are locally owned franchises and you can support a local business that pays fair wages by buying locally (rather than the online sellers).
As an added suggestion. Use a double layer of whatever you choose to use. Thick string ends can be real pointy and still tear through the thin tapes. Cheap insurance.