Can I use Forest Seal Stone's "Star Alchemy" Ability if Garbodor's "Garbotoxin" Ability is activated?
Yes, you can. Star Alchemy is not an Ability the Pokemon has, but it's an Ability the Pokemon has access to.
Source: TPCi Rules Team (2024-04-18)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_156
SWSH12_156
XY9_57