Soil changing color

Just a matter of how slowly seedlings drink the water down. Constant moisture leads to growth on top of soil. Algea is of no major concern. Even a little white fuzz is usually harmless as long as it isn't a recent sprout in danger of damping off. Now that it certainly has a tap root all the way to teh bottom, can do a more pronounced wet-dry cycle between irrigations if you want. When you transplant you should be slightly burying the top of the plug, which would take care of the algea. I often brush the top cm or so off as i pot up. too. Letting the top dry between irrigations will avoid it. A larger plant will help drink the water faster and block light (algea needs light). This isn't something you'll have to deal with long-term. it'll go away just by doing things the right way.

Don't worry about, it is just some harmless algae growing on the perlite and/or staining of the perlite from the bark pieces in your soil mix. It won't affect your plant, but it probably indicates that you are watering too often/soil is constantly too wet, which can lead to issues later on.