The radio's USB connection is for mp3 players. That means one storage place and files in the root of the device, or folders with files in the root of the device. It's not made for a smartphone. (I really wish they'd get rid of that in car audio systems - how many people buy mp3 players these days?)
You could just set USB (in the phone) to Charge Only (or whatever Samsung calls it), and the radio won't see anything plugged in. But the charging rate from a connection like that is usually pretty low - a car charging adapter would give you a higher charge rate - and a faster charge if it's Quick Charge 2 or better. (These days you can get a QC 3 for about the same price. If you wait a little longer, you'll be able to get a QC 4 fo not much more, and be set for years.)