My fireplace interior walls have a white powder on them it looks like mildew is on the brick also. How can I get raid of this?

It's called efflorescence. It's caused because there is a water source where there shouldn't be and so when the moisture comes to the surface to evaporate it leaves this mineral behind. Clean it with a product made for cleaning brick or similar cementitious products.

But, you have to stop the leak that is making water come into the fireplace structure. Inspect the fireplace from outside to find the spot of entry.

Jaz

Drylock makes a powder that's mixed with water. Muratic acid which is diluted hydrochloric acid works too.

Do a Google search for MSDS muratic acid You need to use the right personal protective equipment and ventilation.

You need to find the water source.

Just a question. Is this on the inside of your fireplace? "interior wall of fireplace" Sorry, have to ask - just in case. If you have a wire brush, this will also help if it is flat brick inside that area. I another stiff brush and combo with a little softer one for the brick that is not inside the fire pit. I am not going against what KISS said above, but I never advise a newbie to use muratic acid. Even inhaling it in a space not well ventilated without protection is not good. You can get good results without all of that with the proper tools.

If this is efforescense, applying hot water with vinegar is helpful, and getting it wet to soak first, then using a stiff brush to break it up will help.

What kind of brick is it? And where exactly is this? In the fireplace where you burn, or on the outer face of the fireplace?

If you have moisture within the fireplace, then you may have a cracked liner. This does have to be fixed for health reasons too.