Most of the time I'd not be concerned about it. But....why is there a bump over your septic system? That'd concern me and make me think that it's not terribly safe.
LOL, don't worry about the bump, that's how all of the septic tanks are in our neck of the woods!
There are a very few select properties that can have their "load" go downhill and they don't have to have the bump septic, but very few.
We have our septic mound sectioned off and the rest is open to the horses. I too thought/think that the horses should be able to go on it, but, my husband won't allow it. I almost think he asked the septic person when we built, not sure though.
To know for sure I would ask the people who put them in as their business. They would be the ones to ask. We have so much grazing land that we don't need the area we have sectioned off or I would have asked them about it.
My two cents is, if the amount of grazing land you have is only the area where you have the mound, I personally wouldn't do it. That would mean the horse is on top of it without giving it relief. If you have an acre of good grazing land per horse available without using the sectioned off part, you should be fine.
Good luck with your horse adventure. You are starting out great by asking questions!