[...]"morale" is not an easy concept to describe in a small icon.[...]
In the good old Heroes of Might and Magic they used a golden bird as a symbol of morale.
Wings up -- high morale.
Wings down -- low morale.
I don't have the game anymore, so no better image, but it looked something like
this (in the upper right corner).
Morale is yellow in ufo:ai, so that's why I remembered the golden bird of homm, maybe it is suitable?
Or it could be a flag maybe, or a star, if something different is needed?