I want to add to this because I'm going through a similar problem.
I have some star polyps, though not sure exactly what kind. they aren't gsp, but kind of off white in color and a really brighten up under the actinics. I've had them under a dual satellite as well in a 46bow for the past 8 weeks. They are beautiful! However the past couple of days I've noticed only about half of them are out.
My SG is 1.025-026 (with a hydrometer too). My other readings:
Phos .2 (just can't seem to get off this number, even with phosguard)
Trate 2.5
Cal 450
Alk normal
I noticed one of my cucumbers looked skinny today as well, usually he's quite plump. All other fish and inverts seem great. And I have a xenia colony and leather finger that usually tell me if anything is wrong with the water, yet they have never looked better. My xenia seems to grow a stalk almost every week and my leather has outstanding color, very plump and lots of outstanding star detailed bumps all over him. He's very pretty!
I just can't find anything wrong with the parameters yet I'm having a similar problem with my star polyps.