It will likely get harrassed by the tangs, especially the purple, since they're larger, were there first, and are both herbivores.
But since the foxface has venomous spines, it can defend itself somewhat, so you're likely to see injuries on both the foxface and the tangs.
For example, at one point my purple tang had an obvious puncture wound near his lip that was almost certainly from a foxface spine.
I imagine he lunged at the foxface and got spiked in the face for his trouble.
Eventually the tang and foxface came to an understanding and there's been no issues since.
Also, the foxface will eat the ulva but you can't count on it to eat intact bubble algae spheres.
My foxface has never touched an intact one, ever.
But if I break them off and rupture them with a tweezers and let them go in the water, he's right there to gobble them up instantly.