The Clethra should not be the direct source of the residues, but we cannot say for certain what the causal agent is without more information. Can you share images of the residue you're seeing? Is there wood above the staining that may have been hosting carpenter bee nests? Is there bark mulch in this planting bed?

Our main suspect given the information so far is artillery fungus spores, though they tend to be fairly dark brown instead of yellow-brown. These fungal spores are harmless in terms of concerns over plant infections or structure damage, but as you noticed can be a bear to remove. These fungi tend to thrive on bark mulches, and if the Clethra were mulched with a new mulch supply (compared with what may have already been used on the rest of the plantings in the area) that might be where it originated. (It could also be coincidence, as other factors could be contributing to an "outbreak" of artillery fungus in this one spot. For example, Clethra appreciate regular moisture, as do many fungi. Windblown spores can also easily colonize new areas with conditions suitable to growth.)
https://plantpathology.ca.uky.edu/files/ppfs-gen-06.pdf

Miri

Thank you for the additional information and file. Unfortunately, the photo was not received (our system error says the file is too large). Can you please try re-sending? You can either attach the file separately or include it as part of your reply (in the body of the message).

Miri

The file is too large again. The system limit for attachments is 8MB, in case you need to re-size the image.