Is moisture harmful to a Newtonian Reflector? It is often cold outside, especially in the evening. So when I take my Newtonian Reflector back in my house, it starts condense.

Can the water destroy my telescope? or the mirrors? Do I have to clean it?

My telescope is fully painted, but I'm afraid of the screws. Screws often rust.
The tube itself is not that much of a problem, just wipe it with a soft cloth. The screws should be stainless steel, surface rust can be wiped off as well.

But moisture, which contains humic acids from plants and pollen as well as mold damage the reflective surfaces of mirrors. If it is an expensive one, leave it outside or in a shelter until the next day and store it with the main mirror pointing down.  Don't expose it to a temperature change of -10 to +20°C. Do not clean optical surfaces mechanically if your unsure about the coating and the cleansing stuff. Wiping the aluminium surface will scratch it if it has no protective coating.

If it is a particular expensive bucket and you're in an area that experiences much moisture (where i live, water sometimes runs off the tube, really), consider heaters. Or try another day :-)

Amateur astronomer forums like e.g. cloudynights.com have much more info on this.

Hope that helps.

Edit: also: How do I remove fungus from a telescope mirror?