The urchin has stopped doing it now. I asked some other people on here and they said it is probably spawning due to stress.
Urchins are gonochoristic (meaning there are male urchins and female urchins - no hermaphrodites) and they don't reproduce asexually, so they can't reproduce by themselves.
OP, I would guess your urchin is female, but I don't know for sure.
"The sexes are separate in sea urchins and reproduction is accomplished by shedding eggs (which can number up to 2,000,000 for one female) and sperm in the water"
Source:
The Green Sea Urchin in Maine, Fishery and Biology - a general summary by Stanley Chenoweth, DMR | Department of Marine Resources
This information was written in 1994www.maine.gov
A video by Monterey Bay Aquarium with a male and female urchin spawning (as a note here, the color of the eggs and sperm can vary somewhat, particularly from one species to another - the female in the video is the one with the orangish stuff on it, the male is the one with the whitish stuff on it):
Edit: just to add, it's difficult to find good female urchin spawning videos, but they seem to take longer to disperse the eggs; for the male urchins, they tend to start by releasing just a little bit, then a few seconds later they release a big puff/cloud into the water and they're done.
