Tom M12345

Went to drain my hot tub. When I opened the cabinet, I noticed what looked like light or glowing on some areas of the shell insulation (see pic). Is this normal? Water was still in the tub at this point .I don't think I noticed it before, but it may have been like this. Hot tub is Calspa 5000 model (from 2004). I inherited it when I bought house 6 months ago. Any help would be appreciated. Thanks! Tom