Strange growth on LR

[Abby.O]

Hi everyone,

I'm new to the aquarium hobby and recently bought some live rock for my tank. The tank is only about a week old now and I noticed there has been some rapid growth of some type of hitchhiker on my live rock but I haven't been able to figure out what it is.

It started off as a white/transparent spot on the rock but it's grown to look like some sort of slug with long antennae/tentacles. One has a purplish tint to it.

I know it's not some sort of worm or snail as it hasn't moved an inch in over a week, or when I poke it lol. I'm very curious as to what it might be, and a bit concerned as multiple have started showing up around the tank.

Any thoughts as to what it might be?

 

[Sniperhank]

Harmless filter feeders feeding off the nutrients in the water column. You'll see lots of tube worms and sponges in the early stages of life in a closed system.

 

[Abby.O]

Ok, I was just a bit worried because lots have been multiplying around the tank. Thank you

 

[melosu58]

I agree sponges and filter feeders.