Need help what is this?

[Charphy]

Hello,

 

My tank crashed a month ago. I had a nudibranch in my nano cube. I saw it and it was alive then I woke up in the morning and it was dead along with most corals. The fish store is saying it was probably because of the nudibranch. I did do a water change then another but it was too late.

 

I have been watching the tank and noticed something I haven't seen before. The light were out and I used a flashlight and came across it.

 

Thank you for your help,

 

Kim