"Alien Brain" can be activated when a monster is sent to the Graveyard only if the monster is a Reptile-Type monster both when it is destroyed on the field and when it is sent to the Graveyard. Thus, if the monster's Type is not treated as Reptile while the monster is on the field because of an effect like "DNA Surgery", or if the monster's Type is not treated as Reptile while the monster is in the Graveyard, then you cannot activate "Alien Brain".
If an opponent's Reptile-Type monster attacks and destroys a Reptile-Type monster on your side of the field and sends it to the Graveyard, then you can still activate "Alien Brain". In this case, you still gain control of the opponent's Reptile-Type monster, even though its Type cannot be changed to Reptile.
If you gain control of a monster by the effect of "Alien Brain", then you retain control of the monster as long as the monster remains on the field. The effect which treats the monster's Type as a Reptile is applied as long as the monster remains face-up on the field. Thus, if the monster is later flipped face-down by an effect like "Book of Moon", then the effect which gives you control of the monster is still applied, but the effect which treats its Type as Reptile is no longer applied.