brown diatoms are normal and unavoidable. i expect you are referring to coralline algae when you say good algae. you have to have it present in order for it to grow in the first place. if you have some on your rock then it will spread with the right parameters and light. if you don't have any, you'll need to introduce it somehow.
while i believe no algae is "good" algae in the display, coralline is a calcium based algae and will not grow as fast as other algae. it takes months for it to spread with proper calcium and alkalinity levels.