does anyone know how to remove discoloration around the shower fixtures-----our shower walls are done in venetian plaster.i'm not sure if it's mineral deposit or what. i'm afraid to damage the venetian plaster

What type of plaster is it? Synthetic or lime, resin? The type of plaster you are dealing will effect the cleaning techniques you can use.

Many plasters can be cleaned by a non abrasive neutral cleaner such as 409. Dont use anything with citrus (orange or lemon) and definatly nothing acidic because some plasters are of a basic ph.
I am assuming that a synthetic or a non lime plaster was used on these shower walls and the cleaning technique will vary depending on how badly stained the area is. If formula 409 doesnot work and the stain is deeply imbeded such as a copper looking stain you may need to sand (if possible) the finsh and re-apply.
First and fore most I would find exactly what is causing the problem and correct it or the same thing will happen again (H2O softening system possibly).