if the other mer that gave you kelp has the evil trait you rsim won't become a mer a sto the evil sim it's kinda like playing a trick
is your mod up to date ? the same patch level as your game ? for example if your game is on 1.63 and your mod is 1.55 then the mod is outdated it's good idea to have both at the same level
the other thing you may need to is clear the cache files or edit town save a copy of the sim to the libiary star a new game in IP and try again
make sure it's mermadic kelp your sim eats not kelp
delete the cach files go to electronic arts/the sims 3/
•CasPartCache.package
•compositorCache.package
•scriptCache.package
•simCompositorCache.package
•socialCache.package.package
it's safe to delete these files as a new ones will regenerate