Thank you for contacting Ask Extension. After reviewing the pictures that you sent, your crabapple seems to be experiencing powdery mildew and possibly leaf spot, which is very common in the older crabapples. These older trees do not have the resistance to these diseases, as newer varieties do. Disease resistance is as important as the ornamental traits
of a crabapple. You can spray with a fungicide for these diseases, but you would need to start doing so in the early spring and continue to do so about every 3-4 weeks throughout the summer to keep the leaves for falling off. The tree will continue to have these issues every year because of the humidity and heat that we experience in Michigan. Your tree is not dying, only experiencing a bad year for powdery mildew and leaf spot on them.

The grey and white areas on the branches and trunk are only lichen. This will not cause any harm to your tree.

You can spray the tree, knowing that you will have to continue doing so every year. Or you can take this one down and put in a new cultivar that is resistant to these diseases. I have included a few publications to give you ideas on a new cultivar. My personal favorite is the Prairie Fire. I have had one at the last 5 houses I have lived at and love it. It is resistant to these diseases, has a purply foliage and has beautiful dark pink flowers in the spring. Also, the crabapples are eaten by the birds in February thru March, and not dropped like other crabapples do. 

Good luck and let us know if we can be of further help.

https://hgic.clemson.edu/factsheet/crabapple/

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2987&context=extension_curall#:~:text=Crabapples%20are%20an%20excellent%20addition,in%20many%20shapes%20and%20sizes.

https://bookstore.ksre.ksu.edu/download/flowering-crabapples_MF875