First of all, Genesis does not teach that aliens mingled with humans and created a human-alien hybrid species. Genesis 6:1-8 has been interpreted chiefly in the following ways:
- Angelic beings ("the sons of God") coupled with humans ("the daughters of men"), and the resulting offspring were hybrids between the two kinds of beings. The evil bloodline of these beings had to be eliminated with the Flood.
- Good young men ("the sons of God") became attracted to good looking women from ungodly families ("the daughters of men") and had children by them, and the result was morally depraved generations, which resulted in God's judgment with the Flood.
There's controversy over whether or not children could actually come about from angelic beings, since angelic beings are spirits without a known physical component to their being. Hence the second interpretation I gave. There are more reasons for it (that is the interpretation I hold to) but that's off topic for now.
Either way, there's nothing about gray aliens in there. There are those Christians who claim that encounters with gray aliens are demonic encounters, but that doesn't affirm that demons are physical gray aliens. Rather, it takes into account the teaching that even Satan can appear as an angel of light, and that the people who claim to encounter these aliens coincidentally seem to be into New Age thinking, and New Age is associated with demons. So the Christian belief here is that spiritual beings provide apparitions rather than affirming that physical beings such as gray aliens exist.
Second, let's say that the first interpretation with the angelic beings is correct. Since they are not physical gray aliens, we could not infer from this passage that such a hybrid between a human and a gray alien is possible.
Third, if it is possible, there is no teaching on whether or not such a hybrid would have a soul to save.
Fourth, Hebrews mentions the angels, but specifies that Jesus died for all of mankind, not for angels. Since the angels are excluded, we could not say for sure that any other alien races would be included or excluded in what Jesus did. Proof of such life, which we do not have, would require asking some interesting theological and Christological questions, but we do not have it.