"Phage" is a colloquialism for bacteriophage. Bacteriophages are just one class of viruses. Bacteriophages only and exclusively infect bacteria, not corals or anything else.It's important to understand viruses or phages as they are commonly reffered to by researchers now, are omnipresent in coral microbiomes. As posts here demonstrate, the common perception is phages are bad. In reality phages are essential components of an organisms immune processes. There are good and bad phages. There are also phages that may be living in coral microbiomes without causing problems (lysogenic) and stress can switch them to a disease causing reproductive mode (lytic) causing cell death.
As coral microbiomes, which includes phages, are species specific it seems reasonable to expect species specific responses to stress events. Besides stress events like excessive changes in temperature, excessive changes in lighting, insufficient or excessive nutrients (particulate, organic and inorganic C, N & P) the microbial processes in a system in general (aurabiome if you will) are also factors.
The take away as I see it is reef ecosystems are an incredibly complex with layers upon layers upon layers and species specific variables at every level. Quoting Martin Moe "It's not rocket science, it's a lot more complicated." Since there is a great deal still to be learned and we know the microbial processes in a reef system are critical for sustaining corals AND corals are actively trying to promote processes benefical for themselves, to maintain corals sustainably for the decades or centuries they can live it seems we should not be promoting practices we know skew microbial processes especially as we can't easily quantify or identify which species is proliferating.