Fungus or mold would be my guess, but I can't tell from the photo.
Is it concentrated where drainage from the smaller pot would be most concentrated in the4 soil of the larger pot?
What was the smaller plant, did it have mold or very organic soil, and might it have had some build-up of soluble salts and organics before you set it on top of the larger pot and watered it? Was there mushroom compost in the small pot?
This is not experienced advice, just what I would do:
I would remove the smaller pot and set that where it will drain onto something that you don't care about, like an old Tee shirt or towel, and not draining into any flowerbed or vegetable garden. Flush it well, in case something coming out was "bad" (fungi, mold, eggs, salts, excessive organics, whatever). If it turns out to be something bad AND it came from the small pot, maybe re-pot that small guy and put the diseased soil into a plastic bag in the trash.
Then I would scrape away as much as I could from the larger pot surface. Maybe replace all the mix I could remove without hacking roots.
Once you figure out what it is, it MIGHT not be harmful, but until then, I would reduce the risk to the cherry blossom tree. The longer whatever-it-is stays in the cherry blossom tree pot, the more likely it will eventually make you re-pot the tree.
