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Citrus grown from seeds can take many years to get big enough to grow fruit, but if you are willing to be patient, this article will help: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-seed-harvesting.htm

Yes, this is a problem. Factors that could be affecting it are temperatures, possible cold damage, how long since you planted it, water, even diseases or pests. This article will give you some basic information: https://www.gardeningknowhow.com/edible/fruits/grapefruit/growing-grapefruit.htm

Grapefruit harvesting normally takes place in fall. When the fruits are yellow or golden in color, then they are ready for harvesting.

The following article will tell you some general facts about grapefruit: https://www.gardeningknowhow.com/edible/fruits/grapefruit/grapefruit-tree-not-producing.htm. Grapefruit don't reliably fruit for several years. Continue a good program of care, and your tree should start to bear fruit when it's ready to do so.

The green film is likely algae or mold but the black mold is sooty mold and is common on citrus trees. While unsightly, it's usually harmless. The mold grows on the sweet honeydew (which could also be attributing to the greenish mold) from sap-sucking insects like aphids, scale bugs, etc. You'll first need to treat the offending insect pest (hosing off plant and using garlic spray, insecticidal soap, or horticultural oil). We prefer neem oil, which is safe and effective. As a bonus, neem oil also make a great fungicide, so it will usually take care of both issues. Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm, https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm

It is likely a nutrient imbalance. Try adding more phosphorus to the soil with a high-phosphorus fertilizer or bone meal and see if this helps. For other reasons your tree may not be fruiting normally, read the following article: https://www.gardeningknowhow.com/edible/fruits/grapefruit/grapefruit-tree-not-producing.htm