Mysterious Matter!

Hi! I’ve done a 22 Litre water change (200 Litre tank) and the fish are looking particularly happy today…

This evening I am mostly boring my family because I’ve spotted a small patch of mysterious matter on the front of the aquarium (yes, it’s on the inside!). I’ve tried to photograph it against the moss ball so it shows up. It is only a few mm wide and reminds me of a cross-section of a brain. My wise mum says it looks like a jellyfish… What is it??? Bacteria? Mould? Is it fascinating? It is to me!