I don’t think getting off the horse accomplishes anything constructive. (BIG disclaimer here: if you’re getting off for your own safety, that’s different. Not what I’m talking about. Sometimes on a green horse you just don’t yet have the tools established to deal with Really Scary Stuff.).
In general, I think that if your horse (pick one: spooks, runs away, walks sideways, bulges, prances) when you pass a scary thing, the problem is not the thing. The problem is that the horse blew through your aids, and you can’t solve that problem from the ground.
You can certainly desensitize the horse from the ground. Like with the OP’s scary boulder. You can desensitize the horse to the scary boulder, but the next really scary thing will produce the same result unless you start enforcing the horse’s response to your aids. You can’t desensitize a horse to everything.
Now, I’m NOT saying that you should force your horse into scary situations or put yourself at risk by staying on when you should have gotten off. Ideally, you would practice making the horse respond to the aids despite scary things, starting off in a somewhat controlled environment with a lower intensity of fear and gradually building the intensity.
For example, my horse used to be deathly afraid of mailboxes. He’d prance past them sideways, snorting. :lol: But the problem WASN’T the mailboxes, really. The problem was that he ignored my leg when he was afraid. We worked on that at a lower intensity (starting out farther away from the mailbox), gradually increasing the intensity until I could ride him straight past a mailbox without him bulging against my leg. And that worked for every mailbox on the road, as well as the garbage cans on garbage day, and the trash in the weeds, and the scary rocks, and the neighbor’s unicycle, and…well, you get the point. :lol:
If I hadn’t worked on the aids, I would have had to get off and desensitize him to each new thing. And I am way too lazy to get on and off my horse that much! :lol: