Avoid air bubbles when pipetting the protein and solutions. This bubble popped and then collapsed in upon itself.
growing in the drop looks like this at the beginning.
Mold with skin formation on the drop. The drop is drying out.
The drop has dried out, as seen from the skin formation on it. The mold has started growing on top of the skin. These hyphae growing out of the drop are not a pretty sight.
Work in cleaner conditions. Filter your solutions with a 0.22 micron filter.
Mold and crystals
Crystals can appear in a drop colonized by mold, as seen here (two crystals). However, the protein has often been cleaved by proteases secreted by the mold. Your crystals may not—indeed probably do not—contain the full-length protein.
Run a gel on the crystal to see what the crystal really contains. Or take the crystal to the synchrotron, but bear in mind you may not have the protein you think you had. Remember that when you try to do the molecular replacement.
A piece of cellulose perhaps that has fallen into the drop. Clothing fibers are also common.
Wear a lab coat and work cleanly.
Crystals growing on a fiber
With that said, crystals can often be seen growing on a stray fiber that has fallen into the drop, as seen here.