In that tweet about 'new music' Stevie Nicks was talking about the new compilation album of her solo material called 'Stand Back' that's just been released. The tweet even included a link to purchase the album. She does not appear to be referring to any new material from Fleetwood Mac.
There's a lot of scepticism on the Fleetwood Mac fan site (the Ledge) on whether the band will ever record any new music. Apparently Mike Campbell has indicated he will be devoting next year to touring with his own band.
Certainly I am not aware of Neil indicating that he has written any new music for Fleetwood Mac. He did mention that Mick Fleetwood occassionally sends him snippets of experimental bits of music which he said may one day emerge in some form - but that's about it.