It's probabley not the nutes, unless they've gone bad. Kinda hard to miss. Botanicare stuff does have a lot of sucrose in it, so it can go pretty bad- but it's usually tough to miss.

It's a bacterial infection- otherwise known as rot. The slime on the outside is just whatever psuedos happen to have got hold first- not great, as they block gas exchange and inhibit water movement, but not usually world ending.

I associate that particular grey-green-black w/ anaerobic infections, but that's really just a gut.

Didn't realize Deja had gone DWC- How much mass is he trying to support on how many air pumps?