I’m not actually convinced it’s aphids. But if you think it might be aphids, then here’s what you should be looking for:
If it’s aphids then you will actually see the small insects crawling and or flying around. Aphids are very small, come in many colors (red, green, white, brown, clear, etc…) and are typically 1/4 -1/2 cm in size. Check your leaves for the actual bugs. Aphids often like the underside of leaves so also be sure to check there. If an aphid infestation is well developed, often all that is required to detect them is giving your plant a gentle shake. Signs of aphid activity also include a sticky substance (a byproduct of their feeding on the plant’s sap) which frequently attracts ants, so if you see ants on your plant, you may have aphids.
If you do see signs of aphids, then aphid removal is fairly straightforward, but needs to be done quickly. Aphids can reproduce sexually or asexually, developing reproductive abilities within 1 week, and laying up to 5 eggs a day for 30 days. Left unattended, an aphid infestation can get out of hand very fast.
To get rid of aphids use a small handheld sprayer with a mild dish soap and water solution (lots of recipes online or you can buy at your local garden store). Spray the plant daily, being sure to spray under the leaves as well as on top. It’s important to spray the actual aphid as the solution needs to make contact with the aphid while wet in order to kill them.
If you don’t see signs of aphids, it could be nutrient related or watering (but I don’t see your leaves bending/drooping, so over watering is not the first thing that comes to my mind).
One last thought… I know this doesn’t help your present situation, but some growers feel that the composition of Miracle Grow soil might not be the best choice to get your plants off to a good start (a lot of what I’m reading would indicate people feel that the fertilizer levels within the product are not well balanced to the needs of a marijuana plant). I mention this only as something for future consideration.
PS. If it’s not aphids, then not to worry…there are a lot of very knowledgeable growers in this forum who will probably be able to shed some light on this