In the game of Hearts, when holding J432♥. Does passing a single low heart reduce the risk of someone shot the moon?
No it is not and in fact it might ensure that they do shoot the moon. IF they happen to have the AKQ♥ you have lost the ability to stop it by taking a trick with the J♥. If you pass one of your hearts they are still guaranteed the first 3 heart tricks which would include all 3 of your hearts. Depending on the card distribution that could also leave them with the only heart left to be played.
If you are worried about preventing someone from shooting it would be better to keep all 4 hearts so you have a better chance of taking a heart trick and preventing them from shooting.
One of the biggest liabilities when shooting the moon is low hearts (2, 3 and 4 in particular) because you have to save them until everyone else is out of hearts. This is different from low cards in other suits, where you can try to play them early and bet on a no-trick hand. If someone is sitting on only a few hearts (say just the ace) and mostly high cards in other suits, passing them the 2 of hearts will almost certainly ruin their plan.
On the other hand, one of the best ways to shoot the moon is with a fistful of high hearts that you can use to draw all of the other hearts. If someone has A, K, Q, and 10 of hearts, passing them the 2 in this situation will actually reduce your ability to stop them, as you will only have three hearts left and they can pull all of your hearts with the A, K, and Q. By comparison, if you had kept four hearts, you could have played low hearts to the A, K, and Q, and then had the J in reserve to beat their 10.
The likelihood is low that a player holds the top three hearts. You can prevent a run by the person receiving your pass; unless they have this rare distribution. However a hand with these four hearts would likely be better off passing other cards. Holding these four hearts already makes it more difficult than usual for someone to run. Note that even if you retain 4 hearts, you still need to be careful about breaking hearts in order to defend against a run.