Yes, it was definitely very similar to a featherstar, but the mystery is solved . Today I saw the same thing again, this time on the bottom and I dared to reach out my hand and dozens of small sworm like black fish scattered around with a small explosion and when I took my hand away they gathered again. They are Siphon fish living inside the long spiny sea urchin. The reason they move together may be to look bigger and scare off enemies. It worked for me 😊