If it wipes off, just wipe it with either diluted peroxide or diluted alcohol. Do it at night, so it will have time to "evaporate" before the sun comes up. As long as you keep wiping, it should be fine. If it were mine, I'd bring it indoors into a sunny window for awhile, or put it in deep shade until it recovers. The seedlings are tougher than they look. I've had to top a 2" tall seedling whose leaves were "melting" just like that, and it not only lived, but grew into a gorgeous bush in time.

My plants occasionally have tiny black dots on newer leaves.. The newest and also the most tender leaves become victims to caterpillars and the "dots" are caterpillar excrement.. Check underneath your leaves, especially along the spine, for clinging caterpillars and then relocate as far from your plants as possible.. They will destroy your leaves and flowers in a heartbeat.. Flushing the leaves and even insecticide spraying is ineffective.. The caterpillars are tenacious little buggers and will be quickly replaced by others if given a chance to thrive and reproduce.. Constantly checking your leaves and flowers is your best solution, in my experience.. And if you don't have time to constantly monitor the situation, take heart with the knowledge that your plants are very hardy and will always replace their leaves and flowers over the course of time.. I recently battled a sudden influx of the creepy crawlies for a week solid and thought that I had everything under control and was shocked to find a miniscule "catt" happily chomping away this morning. .. Another "relocation" took place.. Good riddance to all Adenium chompers!