Algae on rockwool??

[HydroBill]

Hi all. I'm getting my roots wet in hydroponics with a single DWC tomato in a bucket. It's about 11 days since I put it in the bucket and the other day I noticed what I believe is algae on the exposed rockwool. Two pictures here, first one shows complete plant, second one zoomed in to rockwool. The top of the rockwool has turned a bright, almost florescent green color. You can see a small spot on the right that was covered by a piece of the grow medium that's still the original grayish color of the rockwool.

Is this bad or harmfull to my plant? Should i cover the rockwool completely with the grow medium or just not worry about it?

Also, I'm using a 6" basket lid. Will this be large enough for a tomato plant? If I decide to continue further into hydroponics, should I get an 8" or even 10" basket lid for tomatoes?

Thank you. Looking forward to learning and growing here...