I got a juniper bonsai in December and I think something is wrong with it. It's pins are turning brittle and slightly purple. And the core of the tree (where the branches meet the trunk) is a weird brown - it didn't look like this before. I also found a white cotton-like fungus on the soil and the rocks in the water tray the pot sits in.

I have to admit, I know nothing about what I am doing! I keep it indoors out of the direct sunlight and I water it when it needs watering by checking the moisture of the soil.
I would really appreciate anyone's help or comments.
Thank you!