The substance that you're seeing being shot up is most likely some extra zooanthellae that they're ridding themselves of as they adjust to the lighting and nutrient levels. You'll know when clams actually spawn when your water column looks murky and you see both sperm and eggs being expelled.
My tank did get murky for a week randomly.... but since my tank is so new, I think it was probably bacteria bloom. I got freebies on my frag plugs then!
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