Well of you took him out of the hydrometer he'd burrow a little hole and stay in there. Trust me, it can protect itself haha :p
Its hard to get a size reference in that pic, but if he doesnt even have colors yet he cant be bigger then 1/2 inch. At that size just about anything could kill him...
This hydrometer is designed to be kept under water, it can also be used out of the water too.
As soon as the resident leaves it'll be in the bin as I've got a refractometer