How big of a tank was he in before you put him in the 2.5 gal? I agree with wilder, this is white spot (aka ich), and you should treat him. the shaking back and forth can be from him itching. Raising the temp to 84-86, add aquarium salt (1 tbsp for 5 gal), and an ich med. there are lots avaliable, the specific med will depend on where you are.
EDIT: Did the shaking start while he was in your main tank?