Strange white thing - maybe a slug?

[Rach]

Hello, I'm new to this forum but I joined because I have been struggling to find out what this creature in my tank is. It has been visible for about a day and had been hanging onto a bit of seaweed and the rock surrounding it. I have attached a couple of photos I see if anyone can help me identify it. Thank you in advance!

 

[Rach]

Here is another photo

 

[Rach]

And another.

 

[Calappidae]

Some kind of weird cucumber.

 

[Betta132]

Looks like a nudibranch. What is it eating, can you tell? If you're really lucky, you may have a nudibranch that can be fed in an aquarium.
Or it could be an odd sea cucumber.

 

[Rach]

Unfortunately I can't tell what it is eating, and on having a look again this afternoon it seems to have moved again and I cannot find where it has gone now. Thank you for the help guys!