Pool allocators don't walk the list or search anything though. All interactions are only at the list head and O(1), as all free nodes are just that, free and equal.
While the most precise naming might be "a singly linked list representation of a stack", "free list" (https://en.wikipedia.org/wiki/Free_list) is an ancient term of art for this problem.. so much so that wikipedia (at least the version today) even suggests "freelist" all one word as a spelling.
The initial linking together of all free space (also cache friendly, btw) is often called "threading the free list", although this terminology is less standard than "free list" itself.