I do not know much but I will try to help...
My guess is some sort of a young pencil urchin, based on the pattern and shape of the spines? It's so tiny at this point that it's hard to tell for sure. Any idea of the ocean of origin?
I tend to go with innocent until proven guilty (especially with tiny things) because I love biodiversity, but I would watch it for sure with stony corals, especially things like montis. I had one a bit like it 15 years ago that got big and started boring into my rock so I took it to the LFS for trade in. It did not bother the corals much, but did make a bit of a mess when it got big. It would disappear for months at a time and come out of rockwork much larger.
I have a different species now that has grown crazy fast but is completely benign and does not leave a 6" diameter area. It eats coralline but it regrows so quickly that it does not even seem to affect the coralline population. It has stayed far away from corals, but I'm watching it closely. It is an Atlantic species with narrow spines.
Some people will buy urchins to graze algae and house cardinal fish, but some people distrust them and are very skeptical. Pencil urchins tend to be higher risk because of their eating habits. But I am only re-entering the reef hobby after 15 years of fw only, so I will defer to others, as my info may be a bit dated.
Some urchins may chew on acrylic - their little mouths (called Aristotle's lantern) are very strong. Also they can be venomous if you handle them - best to let it get on a rock and just remove the rock if you want to relocate it.
