User blog comment:FairyRaven/Fairy Tail Chapter 390: The Story of a Boy, Review and Commentary/@comment-7418318-20140627093953/@comment-24066070-20140628094332

Or perhaps Ur's ice was ingrained in his cells and memory after being frozen for so long and he can now use it as well.

He then just gave himself the nickname of "Ice Devil Slayer", since Ur's ice magic could kill a powerful demon and due to him now being able to use this ice capable of slaying a demon.

What bothers me about the Ice Devil Slayer magic originally belonging to Silver is the fact that Gray couldn't think of who the Ice Devil Slayer was when Atlas Flame told them about Silver. If Gray can remember his father's voice and appearance, he should also remember his father's magic.

Also, while training under Ur he never said anything like "I'll learn ice magic just like my dad".