Greed and materialism are only mere manifestations of the spirit of mammon. They are only the fruit of the bitterly deceptive root of the spirit of mammon, not the other way round. A person under the spirit of mammon may be someone who is not one that is driven only to accumulate wealth. Mammon is a spirit that seduces people into greed and materialism. The person who is seduced by mammon cannot blame it because he has the responsibility to resist mammon which tempts people into serving the earthly or material, as opposed to the spiritual.
Jesus said in Matthew 6:24: No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon. This means that the unsaved will serve mammon without a doubt. However, it does not mean that unsaved will not yield to the temptings of mammon, and try to serve it because of the desires of their flesh. A person needs to choose between serving God or mammon as the two are diametrically opposed to each other.