This is a frog egg nest. The Japanese green forest frog climbs a tree growing over a pond and makes this foamy nest full of frog egg. As the eggs hatch, the tadpoles fall into the pond and mature. There were 5 or 6 of these nests in this tree. I think this may be the same kind of frog that we hear from our apartment in Hakone. It is a large frog, with a big voice, but the song is different than a bull frog.