Clean the plexiglass window pane with regular window cleaner.
The trim potentiometers will hold their positions and the 555 chip is similarly resilient, but the POLY555 is an analog instrument and will lose tune over time. That's expected! Re-tune normally.
3D-printed plastic is susceptible to warping under hot temperatures. Keep it out of prolonged direct sunlight.
When the POLY555 seems especially out of tune, overly distorted, or doesn't turn on at all, that's a sign that its 9v battery doesn't have enough voltage. Open up the enclosure and replace it.