I.D. help , little black bugs on rock wool root zone 1mm

just about to put the cuttings in to the main room and noticed small black bugs moving about on the rock wool cubes. not on the plants. the biggest is about 1mm and then there are smaller ones about 0.3mm.
Any ideas , can't find anything on the net.

Does it only have four legs?

The picture is a little deceptive, it has 6 legs ...

I've looked and a few things are throwing me off. The first thing is 1mm to .3mm is damn small. The next thing is the round head, the pictures aren't the greatest. You might want to try doing a google search for similar bugs in your area. Some bugs only live in specific geographical locations. Don't post your location on this site, if you want to tell me, you can send me a pm. I don't need to know your town or street address or anything specific just the general area within a few hundred miles. It might not even be related to your location. They could've come from wherever you sourced your rockwool cubes from. I would be washing them off. More pictures the better.

looks like a flea beetle,they cause small holes but can chew up leaves if you have a lot of them.
dust with de wont take much but get a true id first,since the picture sucks lol