Isn't it just a "glass mosaic"? Here: mosaico per piscine in pasta di vetro = glass mosaic for swimming pools.
See also this thread
Anyway, according to the glossary, Glass paste (pasta vitrea) indicates coloured opaque tesserae produced with traditional, almost lead-free, glass.
They explain the technology here:
the microcrystals are dispersed in a coloured vitreous phase ; in other words, these microcrystals are added to the molten mass to make the glass opaque. Historically, in 1700 alessio Mattioli experimentedwith colored glass paste and discovered how to make opaque glass in hundred colores.
Maybe opaque (colored) glass mosaic could be a solution here?
I would say "glass mosaic tiles"
I am not so much in agreement with Tim this time (sorry Tim ), as "molten glass mosaic" sounds like something slightly different.
@Elemika: though I think it's somenting different, very useful pdf indeed!!! Thanks!