If it hasn't been too long, a cotton rag soaked in mineral spirits or naptha will get
it off.

Napthalene? that is very very nasty stuff.

I'm definitely not an expert but I'd probably recommend not using
mineral spirits as it could damage the plastic ('glass'). Better
to use the 90% plus ethanol.

That is our procedure for cleaning my company's equipment. The
glare shield and especially my company's equipment takes tons of
radiation so any imperfection microscopically or macroscopically
could be bad.

Gerald