Yes, MADDERS is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, D 2, D 2, E 1, R 1, S 1.

Is it a valid word?

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

Eurasian herb having small yellow flowers and red roots formerly an important source of the dye alizarin

Also: rubia tinctorum.