Hello, we have a very old, large maple tree in our front yard. Over the last year or so, it's been growing a moss on it and has at least one large hole/cavity (pictured) on it. I'm trying to determine if it is diseased and/or if there is anything I could be doing to preserve it. Thank you in advance.
That's a beautiful tree and we understand your concern.
The growth of mosses and lichens on a tree is not of concern. You can learn more about that here: https://extension.umd.edu/hgic/algae-and-mosses-trees-and-shrubs
The damaged area of one of the trunks is concerning. It looks like it worked hard to heal over the area in the past, but lost the battle, as it appears to be rotting inside, and also signs of boring insects.
The thinning canopy is a sign that all is not well.
Your best bet is to have a tree health expert come out and do an on-site evaluation of all the factors, and to see what can be done.
You can search for a Certified Arborist, who have specialized training and are credentialed by the International Society of Arboriculture by going to www.treesaregood.org
Take a look too at this page from our website that helps people to decide when to think about removing a tree: https://extension.umd.edu/learn/how-do-you-decide-when-remove-tree
At the very least, try to keep it as stress-free as possible. Offer supplemental water if we don't get about an inch of rain a week (a tuna can left out can help you know this, but dump it regularly). You can keep this up until the ground freezes in the winter.