While I won't endorse using any med, other than Prazipro, in a reef... I will say this...
A lot of folks use Chemiclean to rid their tank of cyanobacteria. Chemiclean is basically just erythromycin (the old formula was anyway). The same erythromycin we use to treat gram positive bacterial infections. So, I would conclude that using erythromycin in your reef tank is as safe as using Chemiclean. Of course, how "safe" Chemiclean is... is still a matter of some debate.