Hi, a little light surface rust isn't going to cause any harm to the disc's so it can be left on them. All disc's will develop light surface rust when left damp over a period of time so its perfectly normal. DO not under any circumstances use WD40 on them as it will contaminate the brakes with oil and this is not only dangerous but also can fail an MOT.
If you feel you have to clean it off the best thing to do would be ride the bike round the block with a few gentle brake applications, failing that then use a green scouring cloth to gently remove the rust
