Toxic Spores: catch them and brew Spore Neutralizer

Use the catcher to gather the ingredient

Equip a Bug Catcher at the forest spores and use the current catch interaction. A player who had crossed the Cave of Questions still missed this tool, then posted their own solution: Bug Catcher. Check that your inventory now contains Toxic Spores. The reply does not give a controller button or a fixed spawn rate.

Check the lesson, ingredients and bottle

1. If Spore Neutralizer is missing, check the Summer 4 Advanced Potions lesson and your known recipes.
2. Bring three Toxic Spores and one Lavender to a cauldron.
3. Practice the recipe until Brew is available, then Brew an actual bottle.
4. Return to the blocking toxic cloud. Equip the potion and interact with the cloud, or drag the potion onto it, as the guide describes.

Cloud still blocks the route?

Check the bottle, selected target and recipe name; capturing spores or completing Practice alone is not applying a potion. This answer covers a toxic cloud, not every forest barrier or the separate song the questioner called a de-sporing spell. If this specific cloud remains after application, keep its location and your build for follow-up rather than spending unrelated potions.