Hi Susanne, sorry for the delay in responding to your question! If your starter is developing a skin on top this is an indication that it is getting a little too much exposure to the air. The reason we say to "cover loosely" isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded container and cause the lid to pop off. If won't hurt to cover your starter container securely with plastic wrap or even a lid, as long as there is sufficient headroom above the starter for fermentation gases to collect, and you open the jar now and then to vent the gases. While I'd probably recommend removing the skin, it shouldn't have caused harm either way, and covering the starter a little more securely will avoid the issue altogether. 

Barb

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