Dull colours/dark are pretty usual when scanning slides unfortunately and need to be touched up with software. Remember that slide film is usually around 50ASA and that's slow. I have had a few that are pretty well a lost cause (dull day in the evening etc) - my main camera was fully manual so you had to use a meter of take a good guess (no prizes which one I used). :

Moire patterns and noise shouldn't be there.
A couple of stupid questions.
You have the slides the right way around for the scanner? (shiny side/dull side) - I'm sure you have but needs to be asked.
Next, are they glass covered slides or just the film in a plastic mount (glass can create moire patterns).
I doubt descreening would help with slides.
I've not come across this in any I have done so that's a bit of a puzzle (it's usually newspaper photos that have this problem as the pixels don't match the dots in the picture).
What resolutions have you tried?
If the pictures are not sharp (presuming they are in focus etc) that sounds more like a problem with the seating (ie the scanner is out of focus). I've had that when things were not seated properly.
Do a few tests on a slide that's top quality and bright with good colours - taken on a sunny day etc.
Try changing the resolution of the scanner and, if you can, take the slide out of the mount. It may be the scanner is expecting unmounted film which could account for the problem with sharpness and possibly the other problems as well.
Do bear in mind that, with dedicated scanners, you will get what you pay for so expect the cheaper ones to need more work.
Could you post a scan on you-know-where so I could see the problem?
I could seek it out
