Strange Moving Geometric Foam

I found this strange foam on my rhododendron after a rain. 

This plant has never been treated with anything or sprayed with any soap.
Our garden is kept fairly wild and everything is organic.
The pattern of the foam is intriguing and it has a strange "smoky" substance moving around inside.
It lasted for about 20 minutes then dissipated.
Any idea what it could be?





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