help with saltwater fish tank?

[Colby V]

i got a small piece of live rock and ive had it in my tank for about a month and i looked in there the other day and this small thing with fronds waving was on it. at first i thought it was an anenome but i remembered seeing polyps at petco and they looked similar this thing is clearish white and when u put your hand by it it shoots back inside something. if anybody knows what this is i would like to no after further inspection i see that they retract into a white tube