My dad grew up in the Rhonda, Wales. Mooch was often used by him and so I took it as a normal part of speech. Looking back I can't say that I heard it used by others in brum. Having retired I now live in Ross on Wye which is on the Welsh boarder and clos to the Rhonda, Here I find the word Mooch is used.MOOCH Anyone remember this saying.
May be you have someone from wales in your background who introduced it to you family. Please post telling me if this may be the answer to it being heard by you.