Greetings :

The "sickly" tree in your photos is an ornamental crabapple. From your photos I will surmise that the original trunk/leader on this tree was somehow injured when young and died. The cut stub still showing in your photo of the top of this dead section should be cut out to where live tissue starts. The long ugly scar down the side of the trunk will not be possible to fix. The tree has callous tissue started around this area and should grow over and hopefully close off this wounded area in time.

The "lean" on this tree could be caused by high winds with poor rooting. Not sure what rootstock it was propagated on nor how well your soil drainage is. Too wet, high winds or poor rootstock choice can all contribute to a tree leaning in the opposite direction of the dominant prevailing wind pattern. This usually indicates a poor or compromised root system.

The trees in the background of this photo tell me that it is a fairly young neighborhood? Often on new developments these days, they scrape away the native topsoil and sell it before selling off the lots for development. The buyers get stuck with little topsoil on the site. This could be why your crabapple is struggling to grow if stuck in heavy clay soil that doesn't drain well.

The moldy looking spots on the trees trunk are lichens. They don't hurt anything. They can grow on the bark of many tree species.