The moisture sensor on S8/S8+ is a totally new error. I panic when I got it the first time. This was not evident in the earlier Glaxay S series model.
You will not be able to charge your phone when you got the above error. The only way you can charge is by using the wireless charging station if u have one.
Else get ready to do the following. Clean your data cable and S8 charging port. You can use a cotton bud or any soft material that can wipe on the port. This will help to remove the moisture present on the port of charging cable.
If the above does not work, then you will need to visit the local Samsung Store.