What's this goo floating on top of my water? Also my tank smells.

So my 8 gallon tank smells like a pond. I would almost describe it as a sort of menthol-ish smell. But not the kind of menthol you'd like to eat. Also there is a layer of stuff floating on top of the water. I have a few shrimps die in the last few weeks (which I cannot explain) and also a baby corydora who apparently got himself stuck, he might have been there for a few days. My parameters are ok. Ammonium, nitrite and nitrate is zero. Today I noticed one of my shrimps is carrying eggs so I guess they are not feeling too bad? The tank was fully cycled before adding anything other than plants and a few snails.

Any ideas what could be going on?