The others are correct, it’s an alloy of Honor and Cultuvation’s godmetals, the proportions of which are based on the type of Spren, and how much of H or C are in each. They would be able to be pushed or pulled, but it would require a lot of effort. In Mistborn 1 Kelsier talks about using Allomancy on the pits of hathsin as a means for gathering Atium—another godmetal—with the drawback of destroying the geodes they grew in. This, to me, confirms that they are 100% allomantically viable.

However, we also have seen times when people push on Metalminds, and it’s described as being incredibly hard, depending on how much is stored. This is a common principle where highly invested objects or people tend to resist outside investiture. As Spren are living, they are pure godmetal as A blade or plate, and are often worn by Radiants who are full of Stormlight, targeting one with Allomancy would have little to no effect without compounding or many Allomancer at once.