Is the white mycelium often growing on the surface of flower pots beneficial to flowers?

It is not beneficial, for the following reasons.

The white mycelium on the soil surface is mostly fungal matter, there is no doubt about that; there is also a white powdery substance that is a saline-alkaline substance produced when the soil is too alkaline, and none of these are beneficial to flowers. The white mycelium described by the questioner is very likely mold, and mold is mainly caused by the potted plant being kept in too dark, too humid a place with poor ventilation, so the various organic substances contained in the soil produce mold.

Mold is harmful to flowers, mainly because the long-term reproduction of mold causes the root system to become diseased, which leads to root rot, and this phenomenon is very common in flower care. Many people deal with it by simply scraping off the mold on the surface, but this method treats the symptoms and not the root cause; after scraping it off, it may grow back after a while.

Yimu suggests that first you must definitely remove the seedlings from that environment and place them in a ventilated spot, increase light, expose them to the sun to sterilize them, or directly use medication to sterilize them; if the infection is severe, you may need to repot them and change to clean soil. In later care, pay attention to ventilation and keep things dry, and just do not let them stay damp for a long time.