I may be biased, given I am now the happy owner of Algorithmic Art , but here's my logic:
On the one hand, the two niches seem to be very similar, both focusing on sharing art created using algorithms and discussiong the methods employed.
On the other hand, the Generative Art niche isn't just a sub-niche of Algorithmic Art, given it also includes the use of organic (AKA. non computer-based) processes, and thus has more scope in that particular direction, while excluding some techniques and methods of Algorithmic art.
Generative art is most commonly defined as being autonomous systems capable of creating something artistic, where the artists input only really comes in the choice of which pieces to display.
I myself actually create (and generate a small ETH profit from) algorithmic art, only some of which I'd qualify as Generative (The pieces I haven't heavily modified).
Of course, a lot of these definitions are still up for debate and even my own vision of just what is Algorithmic can be put in question.
For example, I envision my Algorithmic Art niche to also accept discussions on, say, robots that create new works by themselves (though with a focus on the code used to make them, the algorithms employed in their decision-making, etc...)
But, yeah, I think both niches can co-exist.
I just have to market mine better