The seedlings are too small to identify. Wait until they get their true leaves and take another picture.
The RHS have one, title is Pests & Diseases. ISBN 1-4053-0064-7. I use it all the time
The moth is a Poplar Hawkmoth,
Could be Magnesium deficiency. The cure is to spray the plant with diluted Epsom salts.
Try feeding them with tomato food it may help
They are both garden weeds and the seed was probably in the compost you used.
You forgot to post the picture
I have Vinca major under my leylandii hedge. It will root wherever it touches the ground so quickly spreads. It also has flowers.
Busy-Lissie, I think you should ask for a second opinion before embarking on such a terrible treatment program
FB, I have only one dane, two would be too much