Yellow Fuzzy Particles in Soil: Perlite or Fungus?

Yellow fuzzy particles in clear pot soil: if white and hard = perlite (harmless). If yellow, soft, squishy with threads = fungus.

If Fungus

1. Replace soil in all affected pots with sterile peat-based mix (or 3:1 peat:perlite).
2. Use pots with drainage holes—glass pots without drainage rot roots.
3. Soil replacement is traumatic; some plants may be lost.

Fungus spores came with soil; warm, damp conditions activated them. Not harmful to humans unless ingested.