Friend or Foe?

[Luucky13]

In all the years I have been in this hobby... I haven't ever seen one of these.... New to my tank as of the past couple of days. Doesn't seem to move... But i'm not sure. Also looks to have tentacles coming out of it.

There are two different ones about 3" away on the bottom of the glass:

 

[KJ]

Labyrinthulaceae.

 

[Luucky13]

Not a heck of a lot of info on the web about it. Are they pests?

 

[KJ]

I don't know.