I'd say it's likely not a huge issue. Things can be slow to get going with mixed fermentation beers. You say the slurry was added to a beer already fermented with dregs from an LEP beer? Did you check the gravity of that beer? It's entirely possible that there's no food left for the Boon bugs to munch on. LEP dregs will have a variety of bugs which could well have eaten all the consumables.
Plus I imagine that the pH of the LEP beer has dropped a fair bit. This could in principle slow the Boonbugz from getting started too...
If it were me, I'd get a gravity reading and keep it O2 free (jet some CO2 into the headspace) and let it sit a bit.
Further down the road, I might consider making up and adding a litre or so of wort to see if anything starts moving...