When Lyta walks into the Vorlon's quarters she isn't wearing a mask due to her alterations by the Vorlons. As she is leaving with the Vorlon she puts on a mask.
She has done this more than once, so it may be to avoid letting a chance witness see her exit without a mask, and thus conceal her alterations.
She has done this more than once, so it may be to avoid letting a chance witness see her exit without a mask, and thus conceal her alterations.