Looks like maybe dead colonial hydroid tubes? If so, and no "fan" comes out the end, then they are likely dead hydroids (good thing). If the "fan" does come out the end, they are similar to Aiptasia in that they sting surrounding corals and
may spread rapidly. Are the tubes soft? Colonial hydroids have soft tubes.
A pic with the "fan" out (they appear clear when the hydroid is retracted or dead):
