Seaslug? or Nudibranch?

Hello! A complete marine novice here! and new to the forum!

I have a 2 month old set-up and I have just come across something that resembles a seaslug? I have tried to ID it but was hoping someone here can help?

I have taken it out until I can determine that its safe, as I have read that some hitchhikers can be harmful?

Mike