Sounds like devitrification (when glass starts to crystalize). Glass meant for fusing is formulated to reduce this – though it still happens. Stained glass generally “devits” more readily.
There are some commercial oversprays (also called devit sprays and overglazes) that may help.
Here’s a link to a recent discussion on fixing it after the fact.
Make sure you are cleaning your glass well and spending as little time in the 1350° – 1400° F range as possible.
Helios Kiln Glass Studio