I.D. Please

[MizTanks]

Rusty brown-only in this one spot, which is high flow. Have stirred it lightly several times but returns within 24-48hrs.
5018

[slapshot]

Looks like diatoms to me. They come and go. If you make a drastic change to your system, including adding or losing fish, coral etc, you can set them off. They usually resolve themselves in a few weeks.