Maybe your scalp is just very dry because you're regularly dehydrating it (once a week) with sulfates and then the rest of the time using (admittedly) less harsh surfactants when there's no need. Why don't you try just co-washing throughout the week and waiting and see if you still get buildup? (You already said you got less.) If and when you do get it, you can use your gentler natural shampoo instead of conditioner. If that isn't enough and doesn't work for you, then use the sulfates but don't use it as often as you were if you can help it. Co-washing seems to help a lot of people balance out their sebum levels. Also I'm not sure if the herbal essences mousse has silicones or polyquats or anything else that will make you prone to buildup but believe it or not water is a powerful cleansing agent all by itself and conditioner and proper scalp massage will help lift a lot of dirt, debris, and product residue from your hair. That's why it's working I believe and you should do what's beneficial for you.
3A - C, HP, ME, HD. (Coarse, High Porosity, Medium Elasticity, High Density.)

CG since Nov. 2012

Poos: SM Moisture Retention + Yucca Baobab, TJ's Tea Tree Tingle
Condish: TJ 'sTea Tree Tingle*, SM Moisture Retention* + Curl & Shine + Yucca Baobab, Yes to Blueberries
Stylers: KCKT*, SM Curl Enhancing Smoothie* + Curl & Style Milk*, KCCC*, FSG*, CJ Pattern Pusha, Curl Keeper
Sealers: Jojoba* or Grapeseed* oil

* = HG