السلام عليكم ورحمة الله وبركاته Ahsanallahu ileykum. Is learning quran from a salafi in a Ashari/maturidi masjid permissible? He acts as an actual teacher in that masjid. Jazakallahu khairan

It is allowed, because what matters is that the one you take your religion from is trustworthy in his creed, not the masjid he teaches in. The masjid is only an issue if being there will affect your deen negatively.