Grease / grease on the shoulder buttons of the PS4 controller?

I've found that on a PS4 controller that has been in use for some time, the R1, R2, L1 and L2 keys have been accumulating grease, or rather a substance that has a slightly yellow to white color. My question would be now what this could be and where it could come from, say or if someone had ever had experience with something like that?

This is dirt from frequent use, I clean my controller regularly with ewtas glass cleaner u.ein cotton swab, evt synonymous times try out.