Covering an above ground pool indefinitely
What you see there is not just water - it's probably already an ecosystem all by itself. If you would examine it, you'll see that there may be thousands of bacterial and other microscopic species there, most of which you wouldn't appreciate living in close proximity with people.

To make sure your new renters aren't killed off by the pool creatures, please make sure it is sealed closed, with no risk of people entering it, falling into it, or otherwise being able to use it. This, of course, assumes that you need it to function as a pool once your renters leave. If you don't care for the pool at all, simply fill it in completely and erase all traces, and you should be good.

Or, you can always make it a fresh, pristine and proper pool, and charge your renters more for the privilege.

But as it stands, it's almost certainly unhealthy, and would constitute a public hazard somewhere.