Is it immoral to not report being overpaid?
Just saw I was paid for 7 hours instead of 4.5.
Should I call up?
It depends. If you want to establish a reputation for complete honest at your job, report it. If you don't, someone from accounting will probably find it some day, and deduct it from a future paycheck.
I will tell you, when something like that happened to me, I didn't report it. Many years ago I got laid off by the same company twice in two years, and got the full severance package for a fairly long-term both times. The whole field I had worked in was vanishing (think typewriter repair), so I regarded the money as a one-time gift from Heaven and spent it on retraining in a new field where I've established a new career. Quite frankly the extra money saved me from a personal disaster, and if they ever ask me to pay it back I will, and it will not cause financial difficulties. But in my mind it was a one-time gift from the Gods or the universe or whatever, and as such it needed to be graciously accepted and used well, rather than sticking to the rules and giving it back.