edit: I found that "Zinc Selenide oxidizes significantly at 300°C", it could be that

If it's an optical window, you could be seeing optical hazing from degradation of the ZnSe with moisture forming zinc hydroxides. If using NOA (2-Naphthoxyacetic acid) at 200C, this could be accelerating the process.

I also read NOA melts at 156C, so I'd imagine that its decomposition point in around 200. Might be making some reactive species there.

I would plan to replace the crystal or send it for polishing. Scarred surfaces lead to scattering, which reduces efficiency. Once it returns, do some careful method development for your analytical process. (Note: I used ZnSe for ATR-FTIR in graduate school, and the crystal we had was heavily degraded by acidic attack. We replaced it and saw great improvement in the optical performance, and within a week, we'd cooked it again. Method development doesn't always work the first time!)

Best of luck with experiments!

Only generic cleaning guidelines are given, since the kind of ('white') contamination/corrosion is not identified ― as it would be preferably. Note, however, that ZnSe is generally attacked by acids, even at room temperature.

You may first try cleaning by gently swabbing the contaminated surface with a polishing microfiber cloth impregnated with some ZnSe-compatible cleaning solvent; such as acetone, isopropanol, hexane, diethyl ether, or chloroform. If that fails, you may still try cleaning by warm water and soap. If you think that etch-polishing is required; you may still consider alkalinized potassium ferricyanide aq. sol.: K3Fe(CN)6 (≤ 30 g) + KOH (0.57 g) + H2O (100 cm3); 1―2 min at room temperature.