I always have a difficult time telling from the picture. Having hard water, I usually have some residue on my plants but you use rain water so this is not likely the cause. It could be residue from fertilizer or minerals but it could also be something else...here are some possibilities....
Eggs of white fly or some other pest?
White Root Rot?
White Root Rot Disease | Home Guides | SF Gate
Could it be a type of mold?
https://herbsathome.co/how-to-remove...n-houseplants/
Fungus?
A Guide to Fungus and Houseplants | WallyGro
– WallyGrow
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