I have an LG-G5 phone and had the same annoying USB pop-up and icon problem after my phone was dropped (very briefly) in shallow (salt) water on a beach, even though I immediately turned it off and removed the battery as soon as possible. I went through extensive steps to make absolutely sure there was no chance of moisture inside or out of that phone and battery pack. I air dried it many times and did much more to remove any chance of moisture. The problem persisted. After searching the internet and seeing many people pointing the finger at the connector, I also cleaned the charging connector with spray air and toothpicks to remove any chance of debris. No joy. I finally realized that the salt water might have left a conductive film or some contaminant on the charging connector, or the blade connector inside the phone where the battery pack connects to the electronics inside the phone. The signals on that connector must be extremely high impedance, so it wouldn't take much contamination to give a false signal as though the connector was plugged into a computer. I used quick drying electronics cleaning spray (see attached photo). I sprayed it into the exterior charging connector port to make sure it dissolved any possible film that might be on the contacts. I also sprayed it on the contacts on the edge card connector between the plug-in battery pack and the contacts on the circuit board. I didn't go any further into the phone. I made sure to use enough to wash off the connector well, and let it drip out into the sink as I sprayed it onto those connectors. That spray dries extremely fast. I left the phone open a few minutes even though the spray dried within seconds. I put it back together and the problem is completely gone! If you try this, make sure to use spray cleaner that is meant to clean circuit boards and which won't leave any film or contamination. Also make sure you've removed the battery and that the phone is off. Make sure it is very dry before turning it on again, so using a quick drying cleaner like in the photo will help speed up the process.
