Hitchhikers good or bad?

[Major_oak]

Okay guys I found this on my live rock and I was hoping y'all could help me with an id and tell me if I should leave it or get rid of it

 

[melypr1985]

I'm not much help here, but I did have something that looked exactly like that when I started my tank up... it died away after a month or two after cycling. Never really found a definitive answer for what it was though.