Issue with rr gold nugget

[the camaro show]

So I bought a gold nugget monti from re during their last flash sale and I’ve noticed  a bleaching or stn on the backside of it. Now I’ve never lost a monti at all and I haven’t had a acro stn on me in over 6 months and I can’t figure it out because it’s not like a flaking stn it’s a very even line. I had it at the top of the tank as rr said they love the light. Today I moved it down a little bit and gave it a idone dip and it might look a little better. Tank is a 4 year old biocube 29. Radion xr15 gen4 on 60% sps ab plus and flow is a mp10 and a hydro nano 425. I also have a apex and it’s looking good