mold on wood aquarium cover

[Soup]

Hi, I have a small 20g aquarium (13 in front, rest is ATO and built-in overflow and nano octopus skimmer).

The hood is wooden and I just noticed growing mold growing on the inside rim. Its had water in it about a month and a half.

How can I protect the wood to prevent the mold? I tried searching but to no real available despite finding on a post on the net regarding varnish. I was just worried the varnish could seep into the aquarium though.

What would be the best way to seal the wood and prevent this?
Would siliconing the underside work?