In most cases, lichen won't harm the trees or plants as it is not getting nutrition from the host. Improving airflow and making sure sunlight can get down in between the leaves/branches is helpful to control overgrowth.
The Bonide Liquid Copper Fungicide is labeled for “algae” on turfgras only and not for treating “algae” on ornamentals. As we mentioned above, the best treatment for lichen is to leave it alone and chemical treatments should only be used as a last resort. Bayer Advanced 2 In 1 Moss & Algae Killer RTS is labeled for lichens per the product label however before using an additonal product, we would suggest contacting your local master gardener for more tips as advice on this can vary by the needs in your region.Bayer Advanced 2 In 1 Moss & Algae Killer RTS could be used safely on St Augustine and hardened bark of trees though the conditions that are making it favorable for the moss/algae to flourish need to be addressed. There is a great article by UGA here that might help.