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What is invisible is forgotten.
Literal
Eye [to-に] not-visible things [topic-は] are-forgotten.
The form 忘れさられる contains an apparent さ-insertion (さ入れ言葉), a non-standard pattern where さ is added before れる in the passive/potential. The standard form is 忘れられる. A nearby reading is 忘れ去られる ('be forgotten away,' from the compound 忘れ去る); the kana spelling leaves both readings open. The aphorism — 'what is invisible is forgotten' — has an out-of-sight-out-of-mind feel that translates straightforwardly.