Hippasteria Sea Star (likely Hippasteria californica)
Hippasteria is a genus of sea star in the family Goniasteridae (which is very diverse), mainly found in the deep sea. These sea stars have five arms, a large, flattened central disc, and are known to be coral/cnidarian predators.
This individual, seen at Hydrate Ridge, was tentatively identified (based on the marginals) by Chris Mah, of the Smithsonian National Museum of Natural History.
References:
https://en.wikipedia.org/wiki/Hippasteria
Mah, C., Neill, K., Eléaume, M. and Foltz, D. (2014), Hippasteria revision. Zool J Linn Soc, 171: 422-456. https://doi.org/10.1111/zoj.12131