Okay so first of all I have never heard term Bielochrovat in my life, it doesn't even sound polish (unlike Krakus and Krakowiak which are well known).
I tried to google some info and of course wiki comes up first
https://en.wikipedia.org/wiki/White_Croats
White Croats, or simply known as Croats, were a group of Early Slavic tribes who lived among other West and East Slavic tribes in the area of modern-day Lesser Poland, Galicia (north of Carpathian Mountains), Western Ukraine, and Northeastern Bohemia. They were documented primarily by foreign medieval authors and managed to preserve their ethnic name until the early 20th century, primarily in Lesser Poland (Lesser Poland is where Kraków is btw). It is considered that they were assimilated into Czech, Polish and Ukrainian ethnos,and are one of the predecessors of the Rusyn people. In the 7th century, some White Croats migrated from their homeland, White Croatia, to the territory of modern-day Croatia in Southeast Europe along the Adriatic Sea, forming the ancestors of the South Slavic ethnic group of Croats.
> Are Poles and Croats lost brothers???
It unironically might be. I was in Croatia and I found it very easy to communicate with them because their language seemed similar, in fact more similar than Czech. I also knew a Croatian dude on internets, we spoke primarily in english of course but sometimes we would start speaking slav for fun and it was the sames.