agression towards other cat

First a little background: I moved to where I currently live about 2 years ago. When I moved here I brought my two cats. Over the past two years 4 other cats have made there way from the forrest that surrounds my house to find my home. I cant turn down a hungry sad kitty so they all became part of my family. The problem: One of the cats that adoped my (Mr. White) is really agressive towards another one of the cats that adopted me (Honey). They both get along with all the other cats really well and I just dont understand why they dont get along. Does anyone have ideas? the other strange thing is that honey became my moms cat and he always sleeps in my moms room and Mr. White is agressive towards my mom but not toward anyone else. Do you think he knows that my mom is honeys mom so he doesnt like her?

Or it may be opposite around, he doesnt like your mom - and therefore not her cat either. Some smell? something she is doing?

So lets try with the standard treatment which usually some husbands must do:
your mom will be chef cook to Mr White. She shall give him most of his meals -all if practically possible, during a period. Give him small meals - so it will be several times each day. Do you let him get out? Let your mom let him out. NOT YOU. Let her sits at his favorit place, if he wants to lay there - he must lay with her. And so on. Soon will he be friendlier to your mom - and therefore hopefully to Honey too.
Besides, you could try with Felifriend.