Yes, there are sea slugs in Mumbai's waters too. This was shot in a pool on one of the rocky shores in the city. This is a new species of Cretana that will be formally described and named soon. It is grazing on a hydroid colony (you can see the flower-like hydroid polyps at the upper left corner. The slug eats the hydroid's stinging cells in such a way that it can arm itself with these cells, and can subsequently use them against predators.
In spite of the bright colours, the slug can be hard to spot because it is very tiny (less than a centimetre long) and has a very specific niche and microhabitat preference.
Canon 760D, Canon 100mm Macro USM
Inbuilt flash fired
Thanks, comments and critique welcome.