Fish biting my urchin

[Caleb1266]

I have a short spine urchin in my tank and my double saddle butterfly is starting to nip at it. Not an actual bite but it seems like he is testing the waters. Should I be worried? What are the consequences of him taking a solid bite out of the urchin

 

[najer]

My guess is that it is nipping at the tube feet, I have a cherub I am trying to catch that does it to my tuxedos.