Yes, SKULLCAP is a Scrabble word!

Letter values in Scrabble US: S 1, K 5, U 1, L 1, L 1, C 3, A 1, P 3.

Is it a valid word?

SKULLCAP is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

rounded brimless cap fitting the crown of the head
a herbaceous plant of the genus Scutellaria which has a calyx that, when inverted, resembles a helmet with its visor raised
the dome of the skull

Not playable in Scrabble US: HELMETFLOWER.