Answer: The mold and barnacles should just be growing on the deecaying mulch or possibly on decaying wood beneath the soil, so should not be bothering your trees. These types of mold or fungus are a natural part of the decay process and so should not be harming live material.
The back discoloration could be caused by canker or other pest or disease problem, but unfortunately I am not able to diagnose it long distance. I would strongly suggest you consult with your county extension, or if you have an extensive collection, you might want to also consult with a professionally trained and certified arborist on site to try to obtain a specific diagnosis of what is wrong with your trees. Based on knowing that, you can determine how to proceed. And if treatment is needed, they will have the most up to date information on what to use and how/when is best to apply it.
You might also want to work with them to try to determine if there is an overall problem affecting your plants -- it could be soil-related such as pH) so you would want to run soil tests (your county extension should be able to help you with how to collect and send in the samples and interpreting the test results.) Or, it might be related to the mulch you are using (for example, it could be too deep or might contain too much fresh wood rather than bark) or to over or underfertilization, or to poor drainage or incorrect watering, herbicide residues or misapplications, and so on. There are many different factors that could cause plants to fail to thrive and it can sometimes take real detective work to figure it out.
I'm sorry I can't give you a specific answer but I hope this helps you begin to figure out the problem. Good luck with your plantings!
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