That’s ma with a dot underneath- bad ghosts. After witnessing the len dong ceremony (details on my main blog) and talking to Kirsten Endres and her friend Andrea it seems to me that there are different purposes for the rituals in len dong to the mother goddess. Rather than spirit mediumship per se it is a chance to worship the mother goddess and share her bounty with your close friends- the ritual involves the handing out of money and offerings to the audience. Kirsten and Andrea said the shamans are not allowed to talk about ma- bad ghosts. Which may explain a lot. I also suspect that the existence of spirits may be taken as lightly and as heavily as all the different continuum of acceptances in the West. Ditto with the practice of Buddhism in this temple devoted to the mother goddess we ate meat and drank rice wine- not Buddhist practices at all!
I went to the sangha afterwards for a dose of calm and mindfulness and I’m glad I did so- it is nice to leave Vietnam having touched my true self once more with the sound of the bell.