mindstuff is different for each sylvana. It is an extremely light fluid and can be summoned by sylvanas in times of extreme stress to form a protective cocoon around them. It is seen as rude to bring out one's mindstuff while around other sylvanas but there are some [like aqua] who are so anxious that they must keep their mindstuff out for a great deal of the time in a day. It is seen as childish among sylvanas.
Mindstuff while not in use is stored in special glands in the sinuses.