Wow, "Blue WIllow" for a plant saucer. Your plants have the life! ;)

Worms or something else living in the bottom of the pot near the drainage hole would be my guess. If the plant doesn't seem to mind, I wouldn't bother with it.

FataMorgana

Looks like worm castings to me but I'd suggest that you evict (rehome) them. Earthworms are great in the garden but in the confines of a container.....not so much. They turn your potting mix into worm poop, which doesn't have the right consistency to maintain a highly porous structure required for healthy container grown plants.

If you do repot your plant, you could take the opportunity to do some root pruning before potting up in fresh potting medium. Try to get rid of as much of the old soil as possible. Do you know what earthworm eggs look like?

J also suggest you use a piece of window screen to cover the drainage holes. Worms ( or millipedes, sow or pill bugs, slugs) won't be able to get inside.