that little bug looks like a fungus gnat. They are quite common. They tend to feed on decaying matter like potting mix. They may eat the roots of your plants or the cotyledons however but they prefer the fungus on the decaying organic matter (potting mix). If you let your mix dry out well, the larvae tend to die off quite quick. They prefer damp soil. One or two isn't an infestation but the sticky strips with pheremones will get lots of adults.
edit> if they are eating your roots, you can expect stunted growth. Again, one or two aren't an infestation |