I got LR from someone and after a couple days had what looked a lot like what chris posted, the closest I could find were colonizing hydroids and after some research put those rocks out in the snow. the OP's pic looks similar to other images I have seen of hydroids but tough to tell from that picture and worth a little more research in my opinion
this is what mine looked like
from what I read they can take over a tank and smother corals, some pull the rock and pull them manually off the rock. some said they seem to self regulate and have never caused them an issue but the posts I read on RC all said get rid of them before they become a problem.
They’ve been in n that corner of that that Little Rock with palys for 6 months and never spread( not saying they won’t) I have seen one that kind looks like yours pop up between some Xenia ( I know another tank killer ) lol
After this storm I’ll just remove the rock and it’ll be added to the pile for acid treatment