Pictures if possible. It sounds like a bacterial bloom, I'd start with getting a API liquid freshwater master test kit. I don't like those strips.
If your parameters are fine, perform a 50% water change and then see what happens. Most if the time it will run its course and be gone as fast as it came.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley