Hi All, I'm very new to bonsai and have had a few issues with my first tree. I have a Chinese Elm that had an aphid infestation so I cleaned them off defoliated as needed and treated the tree with neem oil and it seems to be recovering well and new buds are forming. I have a soil moisture gauge and only water after the soil starts to get dry, I mist the tree daily and keep some water in the humidity tray regularly. I have it under grow lights and it's toughly 62-65 degrees F with about 50% humidity where I keep it. I keep noticing the small piles/lumps of soil appearing around the tree and I have no idea where they come from. I don't know if this is an insect I'm not seeing or something that the soil does on its own. I'm hoping someone on here can let me know what is happening and if I need to do something different. I have included pictures of where I keep the tree and some pictures of the soil. Any help is appreciated!