Even if they are highly developed, we have spaceships loaded with thousands of WMD warheads ready to nuke them down. Colonization is a way for human to expand in land and everything else. Bring explorer first, everyone knew that. If there are sentient, we should take care of them, put them in cages in the research station or some thing else, take their planet as all cost. Remember how British Empire treated Native American. Human democracy is just for human and its citizens, not for cows, dogs, apes, ...
Non-sentient life, even multicellular. We should be mindful of and study the ecology since that is important for resource development and also research, but it's still morally OK to colonize.
Sentient life, it depends. I would say the first rule is that they were there first so the planet belongs to them. If they are cavemen or have ancient civilizations we could land there and have a colony and even assist them (carefully and considerately) in their development, but we would have to regard ourselves as guests.
If they have well-developed civilizations and borders and all that then we could make contact and humans could live among them (if they choose to allow us), but we would be living among them as immigrants or visiting as tourists or merchants or college students studying abroad and we would have to follow their laws while we were there.
If the life on x-planet is sentient, then we shouldn't encroach on their living space in 99% of cases. The only acceptable time to do such a thing is if Earth had some kind of catastrophe that meant that we faced imminent extinction.
If the life is not sentient, I would not consider it immoral to colonize - so long as we were didn't threaten their ecosystem too badly.
If it's just single-celled organisms, then I frankly don't care.
We evolved from non-sentient organisms. We are the ancestors of bacterium. There should not be an assumption made that due to the current circumstances, where the "life" on the planet is non-sentient, that it will continue to remain as such.
While humans colonising the planet wouldn't seem to be a huge problem on the surface, the chances that we will end up messing up the ecosystem and development of life there is very highly likely (Think about earth).
Therefore, with the possibility of sentient life developing and evolving on x-planet, who are we to try to repress it and prevent it from flourishing? Doing so would be morally wrong.
If life is developing say on Mars and you colonize it, you're probably going to end up killing one of them. We humans are really aggressive, and I bet we could easily wipe out any source of life from Mars. We'll overtake it and take away any possible life forms on Mars.
BTW, I probably screwed up some of the grammar XD
At present, we have no sense of the quantity of extraterrestrial life in the universe, so I don't think we can make any absolute judgements. However, we know from the Bubonic Plague & the European invasion of the Americas that blending biomes has catastrophic potential. I assume that any planet with a pre-existing biome would be at least as toxic to humans as sulfur clouds or ammonia oceans. Certainly, we would require decades or centuries of careful study before attempting to share an ecology with alien life. Certainly, sentience has the potential to increase the burden of care but I am skeptical about whether we have a working definition of sentience, much less the capacity to recognize intelligence in alien milieus. The smart move for human survival is an incredibly slow, respectful approach to any sort of alien life (& terrestrial life while we're at it).