Venomous or not?
Thank you for your question. First of all, your photographs were taken a little far away from the subject to get enough detail. Of course, if you thought there was a possibility it was a green mamba, I understand why you wouldn't want to get too close. This coupled with the variety of the snake population of southern Africa, I don't feel comfortable making a definitive identification based on these photographs.
That being said, I'm pretty sure this is not a green mamba. The green mamba does not have the black cross-bar markings. I believe this could be the Spotted Bush Snake (Philothamnus semivariegatus). This species was formerly known as the Variegated Bush Snake. This is a non-venomous species. This species is common throughout its range which includes Zimbabwe and Mozambique and parts of South Africa, Namibia and Botswana. It feeds on lizards, especially geckos and chameleons and frogs. It is an excellent climber and reportedly often enters houses and other buildings, especially those that have shrubbery planted near the windows.
If you should see this snake again, please do not attempt to handle it based on the above information. I would feel much better if you consult a local authority to get their opinion. I'm not sure where you are located in Mozambique, but here is a potential resource you can contact:
Museum of Natural History
Praca Travessia do Zambeze 104
Maputo, Mozambique, MZ, Africa
Telephone: +258 21 490879
Sorry I can't provide a more definitive answer for you, but hopefully someone at the museum can be of more assistance. If you do get an identification from a local authority, I'm curious to know what species this is, as well. If you could send me an e-mail, I would greatly appreciate it. My e-mail is firstname.lastname@example.org
Just a follow-up to your question. I contacted a snake authority in Swaziland and forwarded your photographs to her. She concurs that the snake in your photograph appears to be a Spotted Bush Snake, not a Green Mamba.
Thanks again for using Ask an Expert.