Please identify this urchin

[DACJ]

I bought this feather duster long time ago, and this urchin was stock in the small piece of rock that the FD tube was hosting on, so I pulled it off since it was too tiny and I didn't want any unknown hitchhiker in my tank, but after I pulled it out I thought it looked like an urchin so I put it in my tank thought it would be a neat addition to the tank with such a tiny urchin.

Within a matter of....about three month, the urchin gas grew dramatically....I've never seen one grow that fast...I have three other urchin large, medium, and small one for each size and never changed in size for a whole year at least.

Two pictures attached, the urchin now, and the ACTUAL size of the urchin on my palm when it was found 3 months ago and now...

A quick ID would be helpful and perhaps some info on it, it only comes out at night and dodge really fast when I use a flash light on it...moves very fast unlike urchins =.=" Just wondering if I should start worrying...Thanks!