Clowns will host whatever makes them feel safe. Also bubble tips are not "natural hosts" to occelarris. Most of the time occelarris will adapt as well as Percs. My onxy wouldn't touch the bubble tip for 4 months. Then I saw sebaes are natural host and they were being hosted within 10 minuets. Thai was the strongest response I've seen with captive bred specimen. If you notice the clowns are affecting the brain coral as hey were doing to my frogspawn you might want to try this.
I placed a plastic 1 gallon aquarium over my frogspawn so my clowns couldn't host it and I introduced a natural host (the sebae) and then removed the cover from the frogspawn and they never went back.