What does a white/yellow rank icon mean
Solution 1:
This a top 500 player icon. That came in the Competitive Play update of Overwatch.
You can check this in Overwatch's blog post of the Competitive Play under the section "Rewards".
The Top 500 icon was confirmed by Blizzard in the Twitter and it was told that a visual bug was recently happening in a thread on the Overwatch forum and in a twitter post (linked in the thread).