gothia,
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You clam is most likely squirting sperm into your tank. I would only be concerned about this if:
* this is a small tank w/ no skimmer - you could need to do a quick water change. My corals actually seem to love clam spawn, but too much of anything can be an issue for water quality.
* "always" means fairly frequently. Under normal conditions, a happy clam will occasionally spawn. Stress, however, can create a spawning event. A few months back, I had to move all 11 of my clams quickly from their malfunctioning tank into my 180 - no time for proper drip acclimation. Two of them spawned within minutes. If you are getting frequent spawning events w/ your clam, I doubt that is a good thing.
You'll need to give us a lot more information (size of clam, size of tank, water parameters, new clam? well established tank? etc. for us to give you the best advice.
Cheri