You can stick your arm in the tank don't worry about it. Just make sure you wash are hands and arms well under water only. I stick my hands and arms in my tanks all the time and my fish have shown no effect.
Also you do not need to remove the decor if you don't want to, since normally waste doesn't get under the decor. If you want to just nudge the decor around while gravel vaccing you can, then just place them back where you wanted them.