I need advice from backyard bird enthusiasts

Mr. B and Mo built me two birdhouses and a bird feeder and bought me a hummingbird feeder for Mother's Day this year. I've had them in the yard for a bit and I have a few questions I'm hoping someone can help me with.

The birds have taken to the bird feeder but today I noticed that I have a lot of odd things growing at the base of the post it's mounted on. There is no question that it is coming from the seeds in the feeder. I don't want these plants.

I know I can dig out what is there now but how can I prevent this from happening again and again? The plants are coming up quite close to some "real" things I have planted in the yard.

Is there a way to attract birds to your bird houses? We haven't had anyone move in.

Does sugar water really go bad? I haven't had a lot of hummingbirds so today I took the feeder down to clean and refill it. While looking up the recipe I noticed that they say the food can spoil. It seems strange that sugar water would spoil so I'm just wondering how necessary it is to replace the food every day or two when it hasn't been used. Should I just add a half cup or so of nectar and clean the feeder every couple of days until we establish some regulars?

And since we're on the subject of such things -- I had high hopes that bats would move into the bat house this spring but we didn't get any bats. Is there any way to attract bats to a bat house?

Thanks!