How do you define "soft reset"? (I dislike the terms "hard reset" and "soft reset" because different people use them to mean different things).
It's hard to guess the cause. It could be that something went wrong with the charging and produced a very unusual effect, but it could also just be pure coincidence - there might have been a failing component or a loose connection and it just happened to go at this time. "Charging slowly" just means "not delivering much current", which isn't dangerous in itself (though might be a sign that your USB lead or the phone socket is failing).
Try booting into recovery or bootloader modes. If the visual effect is still there it isn't a software problem (because those modules are independent of the Android system). And if it's not software that only leaves hardware, in which case the only solution is repair - what that will consist of will depend on what the fault is, but I'd advise backing all of your data up before going down that route. If on the other hand the screen looks normal in one of these modes you could try going into recovery and clearing the cache, or backing up and trying a factory reset. But to be honest I strongly suspect this is hardware.
