Not sure they are aphids, they look just a tad large for those, but the way they've clustered together would suggest they are. It's also unusual to see so many on a plant at this time of year, its more generally in spring on new growth. The other possibility is vine weevil as your tree is in a pot, but I have never seen them cluster together in this way before, and they don't look quite right for those either. As there are so many, I'd be inclined to spray with an insecticide that treats for aphids, covering the trunk and branches thoroughly. Alternatively, if they are only on the woody shoots and not soft, green areas, you can use methylated spirits if you have some - moisten (not soak) a disposable dry cloth with the meths and rub over the affected areas to remove them. See here for general information https://www.gardenfocused.co.uk/tree/japanese-maple/pest.php and image of vine weevil, just in case.