marine tank growth on my live rock

Hey all, I recently moved my aquarium to my new apartment, and I notice a
new kind of organism which has emerged from one of my pieces of live rock.
It is a white milky cocoon looking deal with some denseness in the base of
the cocoon attached to the live rock. It is white with stuff in the middle
like something is inside of it similar to a sac of some sort. There is an
elongated tip emerging from the girth of the organism. Any comments or
speculation as to what this is would be greatly appreciated, thanks!