Great question — answering as the UphillTech team:
Like MaxW said, it's starchy water. This happens because Pasta is made from flour, water, and sometimes egg—that means it’s basically just starch and protein rolled out into different shapes and dried. It’s the starch molecules that are important. Once they’re heated in a moist environment—like your pot of water—the starch will absorb more and more water until it finally bursts. That sends little starch molecules into your water, resulting in white foam. It is not a sign of overcooking. But your picture is a visible sign that there is not enough water in the pot. To avoid it, use a bigger pot and use more water. You haven't mentioned how much water you have used for how much pasta. The...
Full answer by Ess Kay: https://cooking.stackexchange.com/a/92663 (CC BY-SA 4.0).
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