White fluffy stuff can be a sign or mealy bug, which is a white creature rather like a woodlouse in shape but only a few millimeters long. Meths on the insect and also on the white stuff is a good treatment for them but you will need constant vigilance until you have got rid of them. If you have aphids you will be able to see them quite clearly - they are usually green and have wings when mature. Re the bud that has gone yellow perhaps a photo would help one of our orchid experts to help you.

If the "white fluffy stuff" is like very fine spider web it could be red spider mite.
Raise the humidity, and try to eradicate with diluted Provado. Keeping the humidity high around the plants is essential even when bug free.

Many tanks to Meane and also Steragram for your help.

Windmill.

Unfortunately, Meanie, Provado has very little effect on spider mites, and can even make them worse.

Tugbrethil - it did prove very effective on my Mandevilla.
Windmill - take on board Tugs advice, as in all fairness he is more knowledgeable than I am.

Thanks everyone for all your replys and advice.

Windmill.

Hmmm...Imidacloprid does nothing for spider mites over here. Just our luck to have those tough pioneer bugs, too! : D
Seriously, since Meanie had good results, it is probably worth a try, particularly if you have some handy.