It's a while since I've made (vegetarian) moussaka but as Max says in the comments, lentils are a decent substitute here. I've used Puy lentils or lentilles vert*. You want something that holds a bit of texture/doesn't fall to bits too easily, unlike red lentils. A mix of red lentils and lentilles vert works well in other things for giving the sauce some body and maintaining texture. I haven't tried this for moussaka but do use this mixture for lasagne.
For flavour, I'd also add mushrooms, but you don't have to.
Another thing I'd be tempted to try is urid dal (the larger of two things called black lentils) as I have them for making dal makhani. These would need pre-soaking and longer cooking.
*yes, that is what they're called in my English supermarket; they also sell "green lentils" that are paler and larger.