Bought a Rolex Hulk off another member here. Watch arrived and i noticed back of watch case is loose. I tried rotating that backk of the case clockwise but wont turn and remains loose. Scared of water getting in there.
you should definitely fasten it, even sweat can and will damage the movement.
you should use a rubber ball or some friction rag to try to fasten it, or go to a watchsmith and he will do it in seconds with the right tools
A "sticky" ball or special Rolexopener will do the trick, if you don't have a rubber ball, you can use gaffer tape, take a large piece and kinda crumble it with the sticky side on the outside. A rubber glove can also help... but defibnately fasten it as it will damage the mvt when moisture gets in.
If you are turning clockwise and pushing down at the same time, and it won't tighten then there is a problem - the threads are stripped. If that's the case you should contact the seller, as a watch with a case back that won't tighten is virtually useless.