Orange Ghost nudibranchs. Lecithophorus sp. I'm not aware of these guys causing any specific problems. Nudis tend to be very specific feeders. Whatever they're eating they will likely burn through their food source rather quickly and begin to die back. You can remove alot of them by siphoning them out. Keep in mind that many nudibranchs are known to be toxic so a large die off could cause issues. I'd take the same precautions as with flatworms.
Ok nice to know ive ben siphoning them for about 2 months ive probably took out about 15 out so far.Lucky there orange and easy to spot when the blues come on.