I'm used to getting a kind of cracking with Mutsus but not like the picture you show where it goes all the way to the wood. If it had showed up in early spring I'd suggest SW injury.

What is the fertilization history of the tree and lawn? I could imagine a tree doing that because of excess nitrogen and/or water or even as a result of excessive pruning.

If the tree is growing with normal vigor and your primary symptom is a cracked bark I doubt this is a life threatening issue but there should be clear callous already forming to close off the wound.

When asking for diagnosis of a problem you should always mention where the tree is growing and any information you have about the overall management of the tree and the turf it grows in. That also should include whether and with what the tree is sprayed and what the texture and drainage of the soil is.

If the mildew is drastically affecting the trees ability to harvest energy or if it leads to any level of defoliation it could very well be an issue and you should look to control it.