There are two possible causes that I see; iridophores and fungus. Iridophores are the most likely.
Iridiophores are completely harmless, natural spots that appear on axolotls. They are usually on the tail, dorsal fin, sides and head. In a Leucy or Albino axxie they will usually be a faint white or pale gold colour. They will be completely flat.
My way to check is to take a torch and shine the light on the spots. If they reflect back a little (like a reflective jacket would) they're iridophores and completely harmless. The axxie may gain more as they get older and the same trick will work to identify them.
If they don't shine back, you may need to gently touch the axxie's tail and see if anything brushes off. If something does, and it looks a little like wet cotton wool, it might be fungus and will need to be treated with salt baths (followed by tea baths after the salt baths have finished, if you prefer).
I suspect these are iridophores, and completely harmless. As for eating less, axxies typically go through periods of fussy eating as they get older. If he is now a sub-adult he will likely stop eating as often - maybe as little as once every other day. He'll be a sub-adult if he's around 1.5 years old. I usually count 2 years old as being a full adult. I'd only worry if he stops eating completely for four days to a week straight and has other symptoms on top of not eating.