I have made that mistake before, cheaping out on dehydrated block. It needs rinsing till its clear which takes gallons and gallons of water. Otherwise its very high in K and Na which will cause you problems, the Na more than the K.
Easiest way I found is to put it in a pillow case and run a hose or shower through it for about 10mins, if you smash it about its a bit quicker.
Since your already growing in it, flush it, come back an hour later and flush it again and measure the EC run off.
Good coco that's pre buffered straight out of the bag still contains a lot of K and will release it over time, that's why most coco specific nutrients are lower in K. Normally I wouldn't recommend measuring the EC of run off due to this reason, but that block stuff needs a serious flush.
You might not notice an issue in veg, but in flower you will probably end up with a lock out due to salt build up.