Because hydroids rely on food coming to them, they often will cleverly settle near mussels. To reproduce, mussels regularly release larvae into the water, which the hydroid will catch in its tentacles. This ensures a reliable source of food nearby. Completing the circle of life, these hydroids are eaten by nudibranchs, and therefore there’s usually one not far away. Did you find the nudibranch just to the left of the hydroid?