White Spots

Snails don't get whitespot, so it's definitely not that.

Do the spots look like small sesame seeds? Nerite snails lay eggs that look like sesame seeds. They are all over the wood in my tanks. And the filter bracket in my 125 litre. The good side is they don't hatch, and even if they did nerite snails need to develop in salt water so any babies won't survive so no population explosion. The downside is that they don't hatch. They stay there, and are very hard to scrape off. The wood in my tank looks like it has white measles. As your long nosed snails do, by the sound of it. Just because you haven't seen the nerites in that place doesn't mean they weren't there when you were not looking. I also think they can place eggs away from their bodies. My wood has eggs in cracks far too small for a nerite to fit in 

I gave up trying to remove them years ago. Though it is difficult to scrub wood without taking the surface off. But I don't think your long nose snails would appreciate being scrubbed either 

At the seaside you see limpets covered in barnacles. Think of your snails as the miniature freshwater versions of those limpets with the nerite eggs as the barnacles