If I use Sceptile's "Nurture and Heal" ability, is that an extra Energy card attachment or does it count as my one attachment for the turn?
It is an extra Energy attachment. You still get to attach one other Energy card as your normal attachment for the turn.
Source: Primal Clash FAQ (2015-02-05), TPCi Rules Team (2015-02-05)

Relevant cards

Note: All applicable cards may not be listed below.
XY5_9