Just seen something in my tank never seen before!

[taricha]

Mine was not a long-spine (diadema). Was something else. Came on some gulf live rock. May be what the OP has?
Last pic, you can see the back glass he could get to vs how thick the coralline was at the spots he didn't reach.

 

[35ppt]

Redacted

 

[Kuub]

I’ll second 35ppt on Echinometra Sp.
Here is my hitchhiker from gulf rock.

He chows down on Coraline.

 

[SHNICI]

Once I had very similar one, it stay completely the same size for over a year (never ID it), time by time I seen it to munch on the coraline, never done sensitive damage on it (only small spots), the rest of the time spent on the algae in the tank. When I had to change the house I found it dead after few days, and I put it in a small flask with IPA 99.9, I think still have it.
Watch it, that I'll suggest.

 

[SHNICI]

The body is about 6-8mm diameter, the spines about 7-10mm, always was "stucking" like in some hols, was really shy

 

[taricha]

Echinometra sp. maybe?

possibly. Also maybe this?
https://www.kpaquatics.com/product/black-rock-urchin/

 

[vetteguy53081]

Pencil urchin and hard to trust. They either eat algae, early coraline and can grab frags, etc and drag them around the tank.

 

[hawk82]

possibly. Also maybe this?
https://www.kpaquatics.com/product/black-rock-urchin/

Looks like that.