9 is not in the sequence since it is equal to 1^3 + 2^3.

PROG

(MAGMA) lst:=[]; r:=26; c:=r^3; for n in [2..r] do for m in [n-1..0 by -1] do s:=((n^2+n)^2-(m^2+m)^2)/4; if s gt c then break; end if; if not s in lst then Append(~lst, s); end if; end for; end for; lst:=Sort(lst); [p: p in [2..c] | not IsPrime(p) and IsPrimePower(p) and not p in lst];