Answer: Actually, they most likely are fungi. There are a number of "shelf-type" fungi that fit your description well. These organisms belong to the group of fungi we call decomposers. They do not attack living tissues, but instead feed on dead wood and bark, causing it to decompose back into soil. The "oyster-like scales" you see are the fruiting structures of the funji (like mushrooms).
The parts of the tree you are seeing them on are most likely places where the bark has died. There is no product labeled to control them and actually no reason to try to control them. If you keep the tree healthy, fertilized and watered (during droughts), it will increase in vigor and, in time, close over the dead area with healthy bark from the sides.
Thanks for the question!
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