I Pm'd another curly about her experience with Mop and SMU and what she thought might have caused the build up. She says she hasn't noticed any issues with Deva SMU but she had this to say about Mop and it sounds exactly like what happened to me.

I did however have to stop using Mop C because of major build up - even with the low poo. I could probably still use it once every couple of weeks or so (I like the results it gives before the build up starts), but I just haven't put it back in my rotation after getting my hair back to normal after over-using it. I'm not sure what the ingredient culprit is in Mop C, but it took a few weeks to recover from the build up. I even did a baking soda / ACV cleanse, it was awful.
Discovered CG 2007

2C Fia BSL/Thick/Med-Coarse/Low(crown)-Normal Porosity Protein and Oil Sensitive

Link-Spritz & Condish Washing

I'm aCurlJunkieJunky

My hair LOVES Coconut DESPISES Soap Bars

CJRehab or H&B Fix, Deva NO POO, EO Rose & Cham POO & Condish, CJ CurlFix, CJ Pattern Pusha, CJ CIAB, SS FHG, KCCC, BRHG, AG Mousse Gel, ACV Rinses,Shikakai Conditioning Hair Rinse.