Two problems; possible over fertilization and a soft scale insect infestation. High salts and too much fertilizer show margins and tips as brown or browning. The scale must be enjoying the extra nutrients from the damaged edges (?).
This is definitely not powdery mildew. That is always shown over the entire leaf, spotty but not just the margins.
Is there a way to get a magnified view of that white stuff? Could you get a magnifying glass and describe the best you are able? Please give details about what you have done for fertilizer, what type of soil you are using and its formulation on the package...I am assuming you used potting soil, I hope so. I'd also get rid of the bark chips on top. They are not sterilized and could be a source of insects and will mess with your fertilizer. Check to see if your potting soil came with fertilizer as well, please.
Thank you for the additional very NICE pictures. It is scale; one of the soft scales. Cottony Maple scale or even Mealy Bug scale. You've been washing the leaves, yes? Was that black all over the leaves at one point? The black on the margins could be honey dew, rather 'infected' honey dew where bacteria are causing the shiny honey dew (excrement of scale) to blacken. Or you might have also over fertilized and that would make your plant more susceptible as well as accounting for the blackened margins where the extra fertilizer is being enjoyed by these insects.
I am not sure how large your plant is so there are a number of ways to approach this problem. My favorite since you have already been washing this plant and the infestation is going strong would be the use of NEEM. If your plant is out doors wait until dark and spray your plant heavily, until it is dripping. Get the undersides of the leaves as well. Wear gloves. NEEM is considered a 'safe' ISH pesticide and 'organic' but you'll smell that it ain't ivory soap.
If your tree is small enough, I like using a dunk of NEEM. Mix the correct amount in a big 5 gallon bucket using 3 or 4 gallons of water. Hard to tell how big your pot is but if it is smaller than a paper plate, using a paper plate that you've cut one line down the radius to the center, slip the plate around your plant upside down of course. Then holding the plate with one hand firmly to your pot to prevent the soil from falling out, lift the pot and plant up, flip over and gently immerse the entire plant into the batch of water and NEEM. Careful with the very tip, try not to break the tip. Then gently 'swish' your plant around in the bucket. I'd try to keep the plant in the solution for a few minutes.
Take your plant out and turn upright again. Allow to drip onto the top of the soil in your pot. Wash patio off with a hose. Wash your hands, arms and face. If you do the spray, make sure you wash up after that as well.
MOST IMPORTANTLY...read that label very closely. Measure carefully. Follow those directions. It is probably the best of the 'organic and safe' pesticides for effectiveness. They say on some labels, different 'brands' that this is safe for bees but I do not take chances! A bee gets sprayed by this he takes it back to the hive and bees are not as healthy and vigorous as they once were. Killing bees is really dumb of us humans!
I would probably gently scrape the scale off the bark but not the leaves before spraying. You will probably need to do this at least once more within a few weeks. This is a good time of year to attack your scale. That cottony stuff is a protectant and if scale is allowed to get too thick the efficacy of the NEEM is severely reduced.
Next, the soil. You probably got your scale from the bark chips. Potting soil is sterilized. But now your soil is full of immatures and eggs. I would repot into fresh new potting soil and bag that soil, or dump the old soil into that bucket of NEEM before disposing of the NEEM according to directions.