That is just it. The only reference to that alchemic symbol is about the
devouring part. it doesn't go further than that. Solgaleo isn't green, and doesn't do anything that relates it to acid. The alchemic symbol in question depicts a "green" lion and the use of acid to dissolve metals to create gold and is compared to plants using photosynthesis. That's pretty much it. And that is only referring to the chemical meaning.
Based on that depiction, Solgaleo should be a Grass/Poison type, as acid is a aspect of the Poison type in the Pokémon universe.
More than likely, Solgaleo is based on the physiological meaning of the alchemic symbol, not its chemical approach. The Nemean Lion also has a much stronger basis for Solgaleo's creation, especially if one includes the Steel typing. Otherwise, Solgaleo should have been Psychic/Fire, which its not.
http://atlanteangardens.blogspot.nl/2014/06/alchemy-green-lion-devouring-sun.html
That said, regarding the photosynthesis part, I expect that Solgaleo can learn things like Solarbeam, Morning Sun and perhaps Energy Ball as to emphasize its link to the sun, whether its through TM or through its own move set, whatever it contains. I also expect Solgaleo to learn several Fire-type attacks to make its link to the sun that much stronger. For the Green Lion aspect, its possible that Solgaleo's shiny form may be green, although I think it might be black.