Good points on the light scattering in water.
This article
http://www.advancedaquarist.com/2010/2/aafeature about the contents of skimmate covers a lot of similar territory.
It concludes 44% of insoluble dried skimmate was CaCO3, but because the system was running a calcium reactor, the article just says it could be forams, coccos, or particles ejected from the reactor.
Since I don't run a reactor, haven't added new salt mix in weeks, have large grained sand, I'd conclude that most anything calcium carbonate in my skimmate was from coccos or forams.