Corrosion on positive terminal and cable

Corrosion is a result of many things, from normal natural processes to contact with leaked chemicals. If anything I'd also say its more likely that the cracked cable connector is the result of heavy corrosion. And no, corrosion does not necessarily imply battery leakage, a certain amount of corrosion over time should be expected and chalked off to normal wear and tear. Sulfur in vapors from the battery can certainly accelerate corrosion though. So it's tough to say without more info e.g. pictures, age of the battery and cables, comparison to corrosion of similar parts in other locations, etc.

The crack itself wouldn't cause corrosion aside from creating a bit more surface area for corrosion to take place on inside the crack, so I suppose it could cause the crack to spread further. However you might want to investigate other underlying causes for the crack. For example, mechanical stress strong enough to crack the connector could have e.g. also been strong enough to crack the battery housing around the post. In other words, the thing that caused the connector to crack could have also caused the battery to leak which could have accelerated corrosion. Or any other order of cause and effect. Or not.

I'll try to improve this answer with a better description of corrosion mechanisms when I'm not on my phone, unless somebody beats me to it. In the mean time check out http://www.stabiloy.com/NR/rdonlyres/E5F38E54-48BF-43C1-9415-865B903605EE/0/CorrosioninElecApplications.pdf and also http://benignblog.com/2013/05/why-battery-terminals-corrode-causes-composition-cleaning-prevention.html.