What is your Eclipse version and what are you exacltly doing? Did you use a WildFly 13 RunTime?
I am using Wildfly 13 with IDE Eclipse Oxygen.3a, using Wildfly 12 server connector.
I am not using Wildfly 12 runtime on my maven project.
If I need some library from Wildfly 13 then I looked it at modules path and use it as a dependency provided on my project.
So you selected server type WildFly 12 and assigned a WildFly 13 runtime, correct? I am using Oxygen.3a as well and haven't realized any issue with WildFly 13 so far.
What are you exactly trying to do and at which step do you get this message?
Could you share the full log?
Since this is a warning message and no WildFly 13 adapter is available I'd proceed with this. But to avoid any confusions I'd change the Runtime name before.