I'm sure it's something I've overlooked, but I started playing a Muse and I noticed I have two sets of abilities based on weapon I'm holding. Harp and Chromablaster. Until I'm holding a weapon, hovering over each ability will show in red "Available when equipping {weapon}." If I equip the Harp, the text is no longer red and the abilities that require the Harp are available. If I equip a Chromablaster, the text remains red and I cannot use any abilities (and I appear to be holding nothing - I can only fight with fists / autoattack).
Under Passive skills I do have both Basic Harp and Chroma