Yes it’s safe, I use it. The main ingredient your looking for in gel super glues is “cyanoacrylate.” Anything other than that do not use it. This is best for attaching lightweight things in the tank (plants, very light weight decor) and a little goes a long way. Too much and as has been mentioned, you’ll see the white cured glue which can be an eyesore. I personally don’t recommend it for moss as it can actually suffocate and kill the glued portion. A tiny dot will suffice to anchor it if you choose to do so.
Despite nothing published saying it’s safe, I’ve also used Gorilla glue (original) for gluing heavy things like hardscape with no issues either. Follow the directions and just to be safe, I let it cure for a week just for peace of mind before filling the tank/adding that glued piece to the tank. NEVER had any issues with livestock using this method.
for cleaning the glass, you could also consider plastic razor blades. They are usually used to remove things like decals from car paint and windows. They are sharp enough to remove things from The surface, but not enough to etch into the glass like a true razor Blade could. Also, less chance of cutting yourself too!