There are some species of BGA/Cyano that are toxic to livestock in large amounts, the Oscillitoria is not, that's the genus we have. It also does not form heterocyst, so does not fix Nitrogen gas, not that the NO3 is even low enough to force it to go that route in our aquariums(Plants rot and die and leach out N before that range is reached).
I've seen tanks covered, we likely all have over the years.
Fish seemed okay to me. Neglect and algae are two distinct things.
You can have algae, but not neglect the tank. But in most cases, an algae covered tank is more commonly associated with a neglected tank at/on some level. That, not the BGA, is the main issues I'd argue.