Love to see some pictures of your tank. I've kept a lot of Umbrella/Toadstool corals, and some huge ones, but never had one try and escape from the top of my tank.
What I think it'll do is 'spread', BIG TIME, if it's that happy where it is.......the weight of it will lean it over into the current and the head will continue to expand.
The only 'soft' corals I have had that will grow above the water line are GSPs.......and 'above the water line' meaning out of laziness I experimented with THAT exact scenario, and found they could care less as long as over time, a day or 2, they would be submerged again. ******I do NOT advocate this, I wanted to see what would happen.
All the best,
I've seen them reach the water line and kind of stop expanding that direction so they just sortof stay right at the surface but don't really go out of it. I would move it down personally because it would shadow everything that far up and I don't like the look, but I don't think it will hurt anything.