how to clean substrate when carpeted by hairgrass????

No real need to suction the substrate in a plant tank. If you have build up of detritus in the stems of the hair grass just put your syphon in the grass without pushing into the roots to such it up. Also if you are having a problem with waste and dirt it could be that you are feeding too much to the critters and it is being wasted, you may also need to be more vigilant with pruning and cleaning off dead/dying plant mater.

I can barely pull the eleocharis out of the substrate. The suction of my python has no effect on it has all.

I can uproot hairgrass with my siphon, start higher up and work down if need be. If the gunk doesn't move then pump the siphon end up and down to get some water movement going to knock the stuff loose. If your grass is long and gets tangled you may want to comb it with your fingers so it looks better after this treatment.

As well as overfeeding consider the tank's circulation pattern if gunk builds up in the hairgrass. Perhaps there is hardscape stopping the flow and debris collects at that spot.

Thanks for the reply. :3