They're likely nematodes (harmless), which typically appear in new tanks or in tanks where you overfeed. If they're flat and have triangular heads, they're planaria - they can harm shrimp and can also explode with overfeeding.
How long has your system been set up and running? How long did you cycle the tank?
What product are you using to remineralize your RO water?
If you're keeping Crystals, you'll need to know your water hardness. Pick up a liquid GH & KH kit to test hardness.