Whenever you boil potatoes for mashing, a foam gathers at the top. There's no need to skim off this foam if you use part of the potato water for mashing the potatoes. The foam is starch that cooks out of the potatoes. Amounts vary with potato type and tend to be greater in older potatoes.
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If starch reacts with pan metal, the foam may become discolored. In that case, remove it.