That's not BGA, that some other reddish stuff.
Apple snails are general plant eaters.

BGA can be quite transient, If you have a small amoutn and clean things well, add KNO3 etc, it can go away.

Look, I'm the defender of snails by every sense.
They are not appreciated nearly enough and every one thinks they are plague like algae.

But showing cause is tough, you need to learn to grow the algae well, they could just be mechanically loosing it, not consuming it, many such things occur.

I have tanks that had BGa and it went away without any herbivores of fish present. So the herbivore added there would not have caused it.

Showing cause is tough, I've tried many species of snail, there may be one, but most times its a person tryig to sell them or equating correlation with cause.

It might seem like a hard hiney way to be, but that will show a lot more cause and effect than correlation ever will.

No one would add PO4 if we used that standard.
Still, adding more snails, as long as they do not eat plants is a good thing generally.

Regards,
Tom Barr