it is one of the waders (a Tringa or somesuch). It appears to have yellowy or greeny legs. I haven't got any African wader books on me, so can't help otherwise.
I was thinking greenshank but I didn't know if they were found there. I just looked at a list of Tanzanian birds and they are on there. Are they likely or rare?
Based on the waders in my field guides I'm fairly certain it's a sandpiper, just not sure which. The beak looks longer in this picture than the other so I'm happy to concede it could be a greenshank. However in all my books these birds should have darker grey markings on the head and chest.