Pls help me , found this spiny worm like thing

[Stanley Goh]

Can expert here tell me what is this ? Harmful to fish n coral ? As I quarantine my new gsp , I saw this thing moving .... Lucky have not go to my main tank , if its hazardous I will throw this away. I'm still new reading alot but cannot find related information about this thing online.

[Stanley Goh]

Any good solution for me ?

[GuyFawkes]

anything unknown, flush down toilet bowl. always dip corals with coralRX

[Stanley Goh]

Thanks for all good advices....

[taufiq85]

Looks like a sea cucumber or a nudi....either way it looks ugly hahaha better dun keep.