Hi Geert...
...and welcome to the muddy waters of Egretta identification and taxonomy.
Generally speaking, it seems safer to consider any 'black' Egretta along the Tanzanian coast (including Zanzibar) as an E.dimorpha = Dimorphic Egret (let alone Black Egret, which is very different and easy to ID). But things change as you reach the Kenyan coast, because Western-Reef Heron must be taken into consideration
I imagine you were considering Little Egret because of the 'clear-cut yellow feet' as described in Stenvenson and Fanshawe's, I can however assure you that this is by no mean an ID feature (otherwise it would mean that the Tanzanian Coast is full of dark morph Little Egrets...).
From my own experience, Dimorphic appears a bit longer billed than Little, the bill is also deeper based with a slightly drooping tip (all this rather subtle...) and in my opinion, your bird fits perfectly.
To illustrate the serious ID problems caused by the genus Egretta, I have attached a shot taken near Dar-es-Salaam. At first, I thought the white bird might be a Little Egret because the bill looks noticeably thinner and straighter than the bills of the dark birds (not a photographic artefact as I had the same impression in the field) but when reviewing my pictures, I noticed that one of the wing-covets near the wrist is black....and I know that dark Dimorphic egrets commonly show a few white feathers in this area and vice-versa...so I don't know exactly what this bird is, but I think it's probably a white Dimorphic.
P.S: the sandpiper is a common, the white breast sides aren't always apparent
