I have had this problem before with Granfix and my supplier sent the grout away to get tested. I will be showing him this thread tommorrow. Tried new bucket, floats you name it ,was driving me mad. Also had same problem when using silicon on all joints it just goes black where the silicon seems to touch the grout,looks like its mouldy . It must be something in the grout this is not the first time Ive heard this.I used a dettox mould spray to get it off just spray on and leave . ps heard nothing back from granfix