Translate 表面風光、內裏陰乾 in English
It is not an idiom, but a typical 順口溜 (Jingle)

表面(on the surface)

風光(look good)* short from 好風光(good scenery)- something look good in people's eyes)

內裏(inside)

陰乾(slowly dry up in dry dark place)

It means 'someone seems well off in the public's eyes, but in reality, is flat broke.'