yup thats a diffuser alright! ive been seeing more diffusers of this kind recently.. actually i wanted to try something like that but dont have the resources yet to have one shipped all the way to the phils.. i guess its efficient! might be better if you direct a flow of water over it too ! prolonging the contact of the micro c02 bubbles with the water.
*edit they are also called reactors but more properly named diffusers