The simple answer to your question is no, there is no Arabic name for this particular style of music. The only way to refer to it with a reasonable degree of specificity is to transliterate it داونتمبو

You see, musical styles in the west are mini-phenomena. They are ephemeral, evanescent creatures that arise like an inflammation and die out before anybody had a chance to even figure out what it was all about. The names of these styles are capricious inventions of "cool" crowds that use the name as an artificial glue to bind disparate works that somehow express a certain cultural climate, a new technique, or some other artistic feature with which the crowd identifies. Take a look, for example at how many styles are subsumed under the rubric of "jazz." Also take a look at how many names there are for Latin rhythms.

If the term were intended to express a universal musical attribute that could be found in any music, then one would have to translate the content of the term. Tempo is a universal feature, but "downtempo" as a name of a style is not.