From what I've known so far, the more showy and colourfull nudibranchs are, the more specialized their feeding will be (eg sponges, polyps, flatworms or other stinging organisms) Those that feed on algae usually are less striking (with exception of the lettuce nudibranch ?) The sea hares are one example of algae eating nudibranchs like the speckled sea hare below which will squirt out violet ink when stressed or disturbed.
Imagine your tank being filled with potassium permanganate solution.....