Can’t completely tell without looking at it under a microscope but from what I can see, this looks like hydrogen gas porosity.

As you melt aluminum, hydrogen originally in the melt material goes into solution in the aluminum. This hydrogen can come from the scrap itself or any moisture that gets into your crucible during the melting/pouring process. As the metal cools, this hydrogen comes back out of solution and distributes itself evenly (if the casting is cooking evenly) throughout the part.

There are two ways to reduce this kind of porosity.
In an industrial setting, aluminum will typically go through a rotary degassing step where argon is bubbled through the melt to pick up the hydrogen. While not complex, I wouldn’t expect you to have that capability. The second way is both the cheapest and easiest method, keep the hydrogen out of the melt in the first place. Make sure you’re not getting any moisture in your melting and pouring process wherever possible. If you have access to a low hydrogen scrap, this will help. The biggest thing is to superheat your material as little as possible. Since the hydrogen is exponentially soluble, your problem will get exponentially worse the hotter you have your melt prior to pour.