In Kate Greenaway's Language of Flowers, the entry for Alyssum (Sweet) reads: Worth beyond beauty.
An appreciation of more than appearance
This entry gives sweet alyssum a message that can be read as appreciation of character rather than looks alone. Keep sweet in the cited plant name, since common names in historical books should not be silently broadened to unrelated plants.
A personal message might acknowledge kindness, reliability or the work someone does quietly. Use the association to support that observation, not to imply that a modest flower reveals a person's true nature or that a recipient must share the dictionary's values.
FAQ
Is this a compliment about someone's appearance?
The wording points beyond appearance; explain the quality you appreciate so the message is clear.
Source
Kate Greenaway: Language of Flowers, plant-to-meaning entries