Do Android apps store data in clear? As in the title, 

is there any chance someone with Android phone (5+) could access private app's data via USB storage? 
In other words, **do apps encrypt data by default? **(e. g. Google Apps but also other ones)
It depends. First, if the developer is a good one with a brain he will make the app to do so, then you have to consider which kind of data that is.

Obviously you don't want your settings to be encrypted, it's just nonsensical, they will be stored in a xml file (or even json, again, it depends). But things like chat history, credentials, etc. they have to be encrypted.