Just checked, and it is definitely a hyphen in the database. Probably best just to leave out the hyphen if it continues to cause problems - there is no need to use it in the Welsh here.

I think what happened is that it showed you the answer as Mam yng-nghyfraith and to show you the difference between its sentence and yours, it underlined the space.

So it's not an underline, nor a hyphen, but a space -- which was just underlined to point it out.