Crab and weard white spooky thing ID please

[TherealDusky]

Hi! Got another crab to ID
Found it munching on some algae.

What are the chances my black mithrax crab (already sumped) will kill it?

Also have this weard slug shaped thing:

Uploaded a video too, you can see it move

Thanks!

 

[jonny]

Looks like gorilla crab to me - slug thing looks like it might be a flatworm of some sort but it's hard to tell.

 

[TherealDusky]

Cheers
Yeah, i thought flatworm too, but it's not flat , wich confuses me

 

[Lionfish Lair]

slug thing looks like it might be a flatworm of some sort

I don't know anything about crabs, though.