The Aether Walk skill states the following:
Upon reaching 7th level, at the start of your turn, you can magically step into the veil between the planes as long as you aren’t incapacitated. You can move through other creatures and objects as if they were difficult terrain, as well as see and affect creatures and objects on the Ethereal Plane. You take 1d10 force damage if you end your turn inside an object.
The mobile feat allows people to ignore difficult terrain:
You are exceptionally speedy and agile. You gain the following
benefits:
Your speed increases by 10 feet.
When you use the Dash action, difficult terrain doesn't cost you extra movement on that turn.
When you make a melee attack against a creature, you don't provoke opportunity attacks from that creature for the rest of the turn,
whether you hit or not.
Would the effect from mobile override the aether walk clause when moving through walls?
Best Answer
Yes, if the criteria for Mobile are met you do not have to spend extra movement when using Aether Walk.
You quoted the relevant rules and they do not conflict with each other. You are still passing through objects or creatures as if they were difficult terrain it's just that difficult terrain does not cost you any extra movement on turns you use the Dash action.