Eastern Crowned Warblers are the common phylloscopus breeders in Hokkaido. Another species, Sakhalin Leaf Warbler also breeds but seemingly not in the forests near this pool. This species, Kamchatka Leaf Warbler, is a common passage migrant in late May/early June. It is part of the Arctic Warbler complex that was recently split into 3 species.
Not many visit the pool, just the few passage migrants that happen to be passing through that part of the forest I suppose.