use pvc glue to construct a pvc cover/box out of (relatively thick) white pvc sheet and corner profiles. Bolt cover to wall/floor. Use a rubber seal(thick white PTFE tape?)on all edges of the box to make it water tight once you clamp it down. (this is assuming you don't have a vacuum former, access to a 3d printer or are unable to find something to cannibalise)
tiled/grouted corner box with a little door. Tile door, thick white rubber tape and bolts to ensure it's a tight seal.
leave the pipe exposed, but replace the ugly industrial valve with a nice chromed (and therefore largely rust resistant) one like this.

To be honest, I'd go with the last option, but pop some extra silicone around where the pipe comes out of the wall/floor. That's another likely weakspot for rust.

The tiled concrete solution isn't bad, but I don't get whoever made it cheaped out on the valve.

e: oh and if there's the option to install the valve in the basement or somewhere else, I'd 100% go that way and remove this valve entirely.