Oh No!!
Be careful. A buddy of mine at Boston Reefers (Gustavo) who goes by Neo, has a pink pincushion urchin that STOLE his very expensive ACAN frag and stashed it away in the MIDDLE of his 120 G reef. He had to rip his tank apart to find it.
Make sure that any frags you want to keep in place are mounted before introducing an urchin like this.
D